Mr. Fairbairn was engaged for some years on an
experimental investigation of great scientific interest, in regard to which,
although the results were not published in his own name, he was entitled to
much credit. This was an enquiry into certain physical properties of the
materials of the earth's crust, undertaken at the instance of the late Mr.
William Hopkins, of Cambridge.
Mr. Hopkins was an eminent mathematician and
physicist; but he had chiefly made himself celebrated for studies of a novel
and peculiar character, involving the application of high mathematical and
mechanical principles to the science of geology.
The study of the structure of the earth,
although from the time of Hutton and Werner it had occupied the earnest
attention of scientific men, had involved only deductions and reasonings of
a comparatively simple character. Mr. Hopkins had set himself to
investigate, in a much deeper and more comprehensive manner, the nature of
the mechanical forces and conditions which had been at work in order to
produce the observed appearances; and he had shown that they were as capable
of being reduced to rule and law as the phenomena of astronomy, although of
course the processes were more obscure and the demonstrations more difficult
to obtain. He had published many papers, for example, on the mechanism of
glacier motion; on the influence of mechanical forces on the conformation of
rocks, their elevation, denudation, &c.; on the theories of volcanoes and
earthquakes; on the temperature of the earth; on climate; and many other
subjects involving mathematical reasoning.
In the course of investigation it occurred to
him that it would be possible to extend mathematical enquiry to the problem
of the share that igneous action had had in the formation of the crust of
the earth. There had always been a leaning in the minds of cosmical
philosophers to the hypothesis that our planet had originally been a globe
of matter in a state of fusion, on which a crust had become formed by the
gradual cooling of the exterior, leaving the interior in all probability
still a molten igneous mass. In support of this hypothesis were brought
the well-known phenomena of volcanoes, earthquakes, and hot springs, as well
as the established fact of the gradual increase of temperature in descending
below the surface of the ground.
In January and March 1839, Mr. Hopkins laid
before the Royal Society two papers, i On the Phenomena of Precession and
Nutation, assuming the Fluidity of the Interior of the Earth,' in which were
some profound speculations- and calculations as to the refrigeration and
internal heat of the globe, and their effects on astronomical phenomena.
These were followed by another paper in January 1842, 011 the ' Thickness
and Constitution of the Earth's Crust>' ill which he pointed out that the
problems would be materially affected by the eject of pressure on the
temperature of fusion of the different matters forming the earth's crust,
which were supposed to have been in a liquid state. He added, 'With the aid
of a proper series of experiments on this point, a direct method of arriving
at an approximation to the thickness of the crust of the globe, or rather to
its least limit, might be easily explained.'
At the meeting of the British Association at
Oxford, in June 1847, Mr. Hopkins presented a long and elaborate report 'On
the Geological Theories of Elevation and Earthquakes,' in which the problems
of the fluidity, solidification, form, and thickness of the earth's crust,
were largely treated of, and he again urged (p. 52) the importance of the
experimental determination of the influence of pressure in the process of
solidification. To show in what a remarkable manner this would bear on the
theory of the earth, he said :—
If this influence can be detected at all by
experiment, it is probably considerably greater than stated above, as
sufficient to justify the conclusion of the earth's solidity to a great
depth; and there could, I conceive, in such case be little doubt as to the
earth's entire solidity. If, on the contrary, it should appear that pressure
exerts no such influence, or that it tends to retard solidification, we must
conclude that the interior temperature of the earth cannot be due to its
original heat. Whatever, then, may be the results of experiment on this
subject, they must probably lead us, should they be sufficiently
determinate, to conclusions of the first importance in speculative geology
respecting the state of the interior of the globe.
To obtain these data by experiment, it became
necessary to consult a mechanical engineer; for the means of producing and
resisting the enormous pressures required were far beyond the scope of any
ordinary laboratory arrangements. Mr. Fairbairn was known to Mr. Hopkins,
not only as a practical engineer, but as a man devoted to science, and Mr.
Hopkins, early in 1851, wrote to him as follows :—
Cambridge, April 25, 1851.
My dear Sir,—I am very anxious to get some
experiments made for the purpose of determining whether great pressure has
any sensible effect on the temperature of fusion of any proposed substance
(a metallic substance for instance), or what will probably be found to be
the same thing, on the temperature at which any substance, in a previous
state of fusion, will become solid.
For this purpose I want the means of producing
an enormous pressure on a cylinder of perhaps an inch or rather more in
diameter, such as Mr. Hodgkinson produced on similar cylinders in his
experiments on the crushing forces for them. My friend, Professor Willis,
informs me that the lever which Mr. Hodgkinson made use of for this purpose
is still, as he believes, in your hands at Manchester. And ray object in now
writing to you is to ask whether you could allow me the use of it, supposing
the experiments to be made under my own superintendence, at Manchester. I
have no doubt of being able to pio-cure considerable pecuniary1 assistance
from the committee for disposing of the annual grant made by (xovernme.nt
for scientific purposes, but I should not like to apply to them till I can
see my way clearly to the means of performing the experiments effectively.
With the exception of the lever, the apparatus required would be of very
If you would have the kindness to give me an
answer as soon as may be perfectly convenient to you, I shall feel very much
obliged to you, as my application for pecuniary aid must be made during the
Yours very truly,
Io this Mr. Fairbairn replied :—
Manchester, April 29, 1851.
My dear Sir,—I have a lever such as you
describe, with ail the requisite apparatus, the whole of which is very much
at your service. The strength of the lever is computed to a pressure of
about fifty tons, but this may be doubled by an additional apparatus if
It will afford me great pleasure to render any
assistance towards the completion of your interesting experiments, and I
shall be glad if you will inform me when you can visit Manchester for that
purpose. In the interval you will perhaps inform me further of the nature of
your experiments, and the preparations you will require to render tliem
effective and satisfactory.
I am, My dear Sir,
Mr. Hopkins then applied for the Government
grant, and obtained an allowance of 250. 'for investigations on the effect
of pressure on the temperature of fusion of certain substances.'
It was stipulated in the terms of the grant that
the expenditure of the money should be under the superintendence of a
committee, in which the name of Mr. Joule was inserted. This gentleman had,
as is well known, identified himself especially with the study of the
mechanical action of heat, and he took, thenceforward, an active share in
In July Mr. Hopkins gave some further
explanations, of which a few extracts may be inserted, as illustrating the
nature of some of the difficulties Mr. Fairbairn had subsequently to
One of the most important, and perhaps difficult
points, w ill be the determination of the time when the solidification of
the matter experimented 011 (concealed from sight) takes place, and its
temperature at that time. It has occurred to me that this may be done by
carefully observing the change of temperature, as the matter is allowed
to cool from a temperature which maintains it in a state of perfect
fluidity. The temperature will decrease pretty uniformly till the
solidification begins, but will remain nearly stationary, I conceive, till
the solidification is completed, after which it will again regularly
decrease. This is the first point I wish to have clearly tested, to
ascertain with what degree of exactness the stationary temperature can be
determined. If it can be done with accuracy I anticipate no serious
difficulty in the experiments. He then went on to describe certain
preliminary experiments on easily fusible substances, and added suggestions
as to how these might be extended further, recommending Mr. Fairbairn to
consult with Mr. Joule on the subject generally.
After some further correspondence, Mr. Hopkins,
in the middle of December, visited Manchester, and conferred fully with Mr.
Fairbairn and Mr. Joule; the nature of the apparatus was settled, and it was
put in hand, Mr. Hopkins paying another visit in January 1852, to examine
what was being done.
The construction appears to have occupied some
months, for in April, Mr. Fairbairn, in writing an account of what he was
doing to his friend Dr. Robinson, of Armagh, remarked that he had some
difficulty in f procuring vessels sufficiently strong at an increased
temperature, to retain the substances on which the experiments were to be
Constant discussions went on between the
parties, and on May 8 Mr. Fairbairn wrote:— it. If, however, you can
overcome them, I believe it will be the best arrangement possible. [This was
On June 26 Mr. Fairbairn wrote from London:—
I had no time to write you on the subject of our
experiments, which in some respects were highly satisfactory, in others not
so. The apparatus is now completely insulated and perfectly tight, but we
had some difficulty in reading off the temperature, &c., &c.
The earlier experiments were directed to the
behaviour of a substance easily fusible—namely, spermaceti, the object being
to make very exact observations on the temperature of fusion; and for a long
time the aim was simply to verify by careful observation the suggestions in
Mr. Hopkins's letter of July 1851 as to the temperature remaining stationary
for a time at the point of solidification. When this was settled the
spermaceti was put under heavy pressure, with the object of observing what
effect this had in varying the point at which the congelation occurred. The
experiments appear to have been conducted by all three of the persons
mentioned. Mr. Fairbairn and Mr. Joule were living at Manchester, but Mr.
Hopkins paid, as appears from the papers, ' long and frequent visits,'
during which he was generally a guest at Mr. Fairbairn's house.
Writing to Baron von Humboldt on August 23,
1852, Mr. Fairbairn said :—
I am at present engaged, in conjunction with Mr.
Hopkins, of Cambridge, on a series of experiments to determine the laws of
the solidification of bodies under severe pressure. It is a subject in which
I am sure you take a deep interest, as it involves a question in physics
most difficult to solve ; namely, under what circumstances solidification is
effected at great depths under the surface of the earth, andl how nature
works under superincumbent pressure. I have a powerful apparatus for the
purpose, and can give a pressure of nearly 6,000 lbs. upon the square inch;
but we have many difficulties to encounter, and considerable trouble in
preventing the radiation of heat from the-' vessel which contains the
substances under pressure. I have got the apparatus so far complete as to
indicate with certainty the progressive changes of temperature during the
progress of crystallisation in passing from the fluid to the solid state;
nevertheless, we have still much to do.
In September Mr. Hopkins was again at
Manchester, arranging further contrivances for increasing the accuracy of
the experiments, which it still took two months to carry out; and on
November 23 Mr. Fairbairn wrote to Mr. Hopkins :—
Enclosed you ba^e the results of our experiments
on Saturday last, which on examination you will find a nearer approach to
the stationary temperature than any of those yet made.
Od a consultation with my friend, Mr. Joule, we
have come to the decision to let the matter stand over for a time, till we
hear from, or rather till we have the pleasure of seeing you either here or
in London. At our next meeting something must be determined upon, and I
think it may be desirable to vary the form and character of our proceedings,
in order to arrive at conclusions, that will enable you to deduce your laws.
Some further experiments were made, and Mr.
Hopkins replied on December 21:—
I thank you for the account of your last
experiments. I have no doubt whatever of the temperature of solidification
having been obtained very approximately in both of the last experiments
under pressure. We are manifestly approximating to unquestionable results.
After this, new apparatus was prepared according
to the plans jointly agreed on.
In March 1853, Professor (now Sir) William
Thomson, who had been previously consulted on the subject by Mr. Hopkins and
Mr. Joule, devised an improvement in the apparatus, so elegant and ingenious
as to deserve a brief mention. It had been difficult to find out, while the
heat was increasing, the exact point of time at which the fusion took place.
As the only feasible mode thought of, the spermaceti was enclosed in a glass
tube, through which it could be observed, and the fusion determined by its
loss of opacity. Professor Thomson suggested that a small piece of
magnetised steel wire should be put into the spermaceti before it was
enclosed m the tube, and so placed as to be at the upper part of it when
operated on. A small compass was then placed outside the vessel, in such a
position that its needle might be acted on by the magnetised wire inside.
When fusion took place, the wire, being no longer supported by the solid
material, fell to the bottom, and the moment of its doing so was made
evident by the motion of the compass outside. This was tried in July, and
answered admirably, allowing the containing tube to be made of brass, and so
avoiding the danger of the use of glass under such great heat and pressure.
On August 12, 1853, Mr. Joule wrote to Mr.
Seiford, August 12, 1853.
My dear Sir,—Our experiment this morning was
satisfactory, the needle having fallen at 373, the exact temperature
expected by Mr. Hopkins, and which shows that the temperature of fusion
rises with the pressure in arithmetic progression. The lead box answered
capitally. I have sent a line to Mr. H. to communicate the result.
I am yours truly,
James P. .Jodle.
Mr. Hopkins, remarking on this in a letter dated
August 22, said :—
I had previously heard from Mr. Joule of his
completely successful experiment, for such it has manifestly been. Its
agreement with my calculated result is most satisfactory. In this case the
law is clear, that the increase of the temperature of the fusion is
proportional to the pressure.
With respect to the conductive power of the
substances pressed, we had better leave those experiments till my next visit
to Manchester, or at least till after the meeting at Hull.
Could you send to Professor Phillips a drawing
(it does not signify how rough) of our apparatus? It may be as small, too,
as you please. He will get an enlarged drawing of it made for exhibition at
Hull [British Association meeting] if I should find it expedient, as I think
I shall, to exhibit it at the time of my address.
The subject was brought forward at the. Hull
meeting, and the nature of the experiments and apparatus was explained; but
no written paper upon it was presented, and therefore no record of it
appeared in the published report for the year.
The following letter may be put on record on
Salford, August 24, 18,33.
My dear Sir,—I transmitted to Mr. Hopkins
yesterday, an account of some experiments on the physical properties of
beeswax, which may perhaps serve to throw some light on the experiments on
the alteration of the point of liquefaction by pressure. The results arrived
at are as follow :—
The wax softened gradually until the point of
absolute fluidity, 54° Cent., was reached. The increase of the specific heat
at high temperatures was owing to the heat due to a change of state being
I find the specific heat of wax in the perfect
fluid condition to be .506, and another experiment gave -.501.
Taking the specific heat both at perfect
solidity and perfect fluidity to be -5, I find the heat absorbed in changing
the state of one grain of wax from perfect solidity to perfect fluidity to
be 33-2° Cent, per one grain of water.
Professor Thomson's formula gives, with the
above data, 96 division of the thermometer used in the pressure experiments,
or 24° Cent, as the theoretical elevation for our greatest pressure, the
actual result being 68 divisions, or 17° Cent. The difference is not great
under the circumstances.
I am afraid I shall not be able to get to Hull,
unless, indeed, I contrive to get off for one day.
Believe me, dear Sir,
Yours very truly,
J. P. Joule.
W. FAIkBAlRN, Esq.
To this time the experiments had only been
preliminary —i.e., on substances which had no immediate connection with the
enquiry, but were used to obtain general laws. J-iut, emboldened by the
success of these trials, Mr. Hopkins now proposed to carry them out on a
large scale, to apply them to other substances, and to make use of greater
pressures and higher temperatures. He accordingly, on November 7, 1853, gave
Mr. Fairbairn suggestions for the necessary alterations and additions to the
apparatus, and requested his advice and co-operation thereon. At the
beginning of 185 i he again spent some days at Manchester, but the new
arrangements took some time. In April thermometers were still in
construction (extending to 610° Fahr.), and it was the middle of the year
before Mr. Hopkins could get the new experiments fairly in hand.
At this time the second element was brought into
the investigation. Mr. Hopkins desired to ascertain the capabilities of the
various substances for the conduction of heat, and the influence of
compression on this property. Mr. Fairbairn had accordingly to prepare
specimens, in the form of small cubes, which were compressed under pressures
sometimes reaching to 80,000 lbs. on the square inch.
At the meeting of the British Association at
Liverpool, in September 1854, Mr. Hopkins presented to the Physical Section
' An Account of some Experiments on the Eifect of Pressure on the
Temperature of Fusion of different Substances.'1 In
this he stated ' his great obligations to Mr. Fairbairn for the promptitude
with which, iu the first instance, he proffered his assistance and
cooperation ; and the manner in which he had since afforded the aid of his
great practical knowledge, and the ample means which his establishment
afforded for conducting experiments of this nature.'
Mr. Hopkins described the apparatus, and the
causes of the failures which had occurred successively, till by degrees
apparatus had been devised and constructed which was likely to prove
successful. He then gave the results obtained, which showed that the
temperature of fusion increased in regular ratio with the pressure. The
following were the substances tried, and the melting points (Fahrenheit) at
different pressures :—
At Atmospheric Pressure
At 7.790 Its. fer ».oi re ill.
Atll,88»lbsi pel fcq-jare in.
On October 20, 1854, Mr. Fairbairn wrote to Mr.
The tubes, cylinder and furnace, and other parts
of the apparatus for the compression of substances, are now complete,
excepting only the electric wires and the float for measuring the amount of
compression. Altogether it is a complete apparatus, and may be used in a
laboratory or elsewhere, with tolerable certainty as to the results. It is
rather an expensive piece of machinery, something above 20/., but it is very
complete, and I make no doubt will effect good and satisfactory results. The
only difficulty will be the working of the wires and the float, but that can
only be determined by a few experiments.
It was decided to make the first trial of the
new machine at Manchester; but Mr. Hopkins could not get there till the
Easter vacation of 1855.
On June 16 of that year, Mr. Fairbairn, writing
to Sir David Brewster, said :—
I am the more anxious to see you, as I wish to
consult you upon the experiments on densities, which I mentioned to you some
time since. Some of them are very curious and interesting, and I make no
doubt, with the powerful apparatus I have at command, that some new facts
are almost sure to present themselves. You shall see them, and at the same
time give me your advice, when you come down.
Mr. Hopkins spent some further time at
Manchester in the long vacation, and afterwards, on October 8, 1855, he
wrote Mr. Fairbairn a letter from which the following are extracts:—
I was prevented writing to you as I had
intended, at Glasgow, by the difficulties which continued to beset me in my
experiments till the last moment of my being at Manchester. It was only on
the last day of my sojourn there that I considered myself to have overcome,
as I believe, the last difficulty, and to have obtained results which I
could rely upon with the more difficult substances, such as tin and bismuth.
With respect to the latter, there is clearly no increase, but probably
a decrease of the temperature of fusion resulting from pressure. According
to our new theory of heat, this ought to he the case, provided there he no
increase of volume while the substance passes from the solid to the fluid
state. I had just time before I left to try the experiment, and assure
myself that bismuth in a fluid state probably occupies tes volvrae than in
its solid state, while such substances as wax, spermaceti, &c., of which the
temperature of fusion is so much increased by pressure, occupy
much larger space in a fluid than in a solid state. This J» all accordant
with theory ; but requires s.till to be worked out with accuracy, which
there will be no difficulty now in doing. This, with some other things, must
remain for iny Christmas visit.
But this Christmas visit never took place; Mr.
Hopkins's health began to fail, and he was obliged to give up any further
active labour in the experiments. In August, 1864, he wrote to Mr. Fairbairn
as follows :—
During the winter I have repeatedly formed the
intention of writing to you. My old enemy, the bronchial cough, attacked me
again in the summer of 1863.....I became very feeble and ill. I was sure
also that my memory for abstractions, and my power of continued application
to that kind of mental effort which, during the latter and best part of my
life has afforded me the most intellectually active and agreeable
employment, was in danger of being affected.
I understand that there is 100/. with which the
Royal Society has accredited me over and above what I have drawn on account
of our experiments. I will see, whatever it may be, that it be paid to
yourself. I wish you also to keep all the instruments, about which T 1hink I
spent myself some 501, or 60?. All this, in addition to what the Royal
Society has advanced to you before, will, I fear, be but a very poor
compensation for all the expense and trouble you have undertaken for me. My
age now numbers too many years to allow me to bark as I have done, and for
the last eighteen mouths or more I believe that I have injured my health by
too close application. I am not strong enough at present to carry on the
experiments, and from what I have stated above you will understand that I am
not sanguine of ever being able to do so. They have already enabled me to
produce two memoirs, which perhaps will hereafter be found of some value to
those interested in the subject; but I fear now that I shall not have
sufficient strength to finish the final memoir -which I had contemplated,
though I have obtained a certain amount of materials for it. But it would
require still much labour to make it complete enough for publication, except
as an abstract.
Mr. Hopkins's prognostications were but too well
founded. Soon after this, his powerful mind succumbed to the great strain he
had put upon it, and he died in October 1866.
In December, 1865, the Eoyal Society wrote to
Mr. Fairbairn on the subject of the balance of money ; but he declined to
receive anything further.
The results of the whole matter were given, in a
scientific form, in a paper read by Mr. Hopkins before the Eoyal Society,
June 18, 1857,2 entitled ' Experimental Eesearches
on the Conductive Powers of various Substances, with the Application of the
Eesults to the Problem of Terrestrial Temperature.'
In the opening paragraph he says :—
I am likewise bound to express in the strongest
terms my obligations to my friends Mr. Fairbairn and Mr. Joule. Without the
aid of the former of these gentlemen I should have been unable even to
commence the series of experiments which I have now nearly concluded; and
among the many ways in which this assistance has been so promptly rendered,
I may mention his having constantly placed at my disposal the invaluable
services of one of his principal workmen, William Ward, without whose
untiring activity and mechanical resources I should have utterly despaired
of bringing my experiments to any successful issue.
This paper contains an elaborate mathematical
discussion of the subject; but a more popular account of the results arrived
at may be gathered from a lecture given by Mr. Hopkins before the Royal
Institution, on May 13, 1859.
After stating the facts, showing a gradual
increase of temperature in descending below the surface of the earth, which
(excluding local anomalies) he estimates at 1° Fahr. for every sixty feet
depth, he formulates the inference from it as follows :—
If a sphere of very large dimensions, like the
earth, were heated, and left to cool in surrounding space, it is shown by
accurate investigation, that after a sufficient and very great length of
time, the law according to which the temperature would increase in
descending beneath the earth's surface, would be that the increase of
temperature would be proportional to the increase of depth, which coincides
with the observed law. Now, according to this law, the temperature at the
depth of sixty or seventy miles would probably be sufficient to reduce to
fusion nearly all the materials which constitute the earth's external solid
envelope; and hence it had been concluded that the earth probably consists
of a central molten mass, or a fluid nucleus, and an external solid shell,
of not more than sixty or seventy miles in thickness, or even less.
This has been the ordinary supposition, not only
popularly, but also generally among scientific men. But Mr. Hopkins goes on
to show that the experiments, of which it has been the object of this
chapter to give an account, throw doubt on the conclusiveness of the
reasoning on which this opinion is founded.
He points out that an important element of the
argument is wanting. It involves the hypothesis that the conductive power of
the rocks which constitute the lower portions of the earth's crust is the
same as that of the rocks which form its upper portion. For if the
conductive power of the lower portions of the earth's solid crust be greater
than that of the thin upper portion of it through which man has been able to
penetrate, the depth to which we must proceed to arrive at a certain
temperature (as that of fusion for the lower rocks) will be proportionately
He then shows that the experiments in question
lead to the conclusion that the conductivity of the inferior portions of the
earth's solid crust, which are consolidated under very heavy pressure, must
be much greater, and may be very much greater, than that of the less
consolidated and more superficial sedimentary beds.
Moreover, the temperature of fusion under heavy
pressure is another element in the argument. The experiments show that in
regard to certain substances that also is much increased by great pressure;
and by analogy, it may be concluded that such will, at least in some
considerable degree, be the case with the mineral matter of the earth's
Judging by the data obtained, Mr. Hopkins infers
that the actual thickness of the solid crust of the earth must probably be
at least about 200 miles, and may be considerably greater, even if we admit
no other source of terrestrial heat than the central heat here contemplated.
Mr. Hopkins then goes on to explain another kind
of argument, of a mathematical nature, bearing on the same subject, derived
from the phenomenon of the precession of the equinoxes, which brings out a
similar result; and he concludes:—
Thus, both the modes of investigation described
lead to like conclusions respecting the least thickness which can he
assigned to the solid envelope of our globe. It must be much greater than
geologists have frequently imagined it to be.
The following paragraph from the Royal Society
paper may be added :—
After the preceding investigations, it appears
to me extremely difficult, if not impossible, to avoid the conclusion that a
part at least of the heat now existing in the superficial crust of our globe
is due to superficial and not to central causes. It should be remarked,
however, that the argument thus afforded is not directly against the theory
of a primitive heat, but only against the manifestation of the remains of
such heat as the sole cause of the existing terrestrial temperature at
depths beyond the direct influence of solar heat.
The conclusion that the earth's solid crust is
so thin as many geologists have believed it to be, as well as those theories
resting on that conclusion, whether of volcanic action or of elevation or
depression of the earth's surface, at least in more recent geological times,
must be in a great degree invalidated.
Mr. Hopkins stated in this paper his intention
of following it with another, in which a further portion of the experiments,
not included here, would be given and reasoned on ; but this he had not
strength to do.
It will be gathered from the above account what
great and difficult questions these experiments were directed to; and
although of course it is not intended to claim for Mr. Fairbairn any of the
more abstruse portions of the researches, there is no doubt that his
excellent practical skTl contributed largely to the success of the