April 6, 1871 | 
NATURE 
459 

Joule’s experiments, even when he did not agree with the deduc- 
tions drawn from them, 2. The subject is of extreme im- 
portance both for the interpretation of physical phenomena and 
or determining what limits are assigned by the stern laws of 
nature to the exercise of man’s mechanical and scientific ski 
3. No doubt Dr. Joule has ascertained the heat ordinarily derived 
from the destruction of energy, by means of friction with various 
substances ; but it has been assumed, 77 defiance of facts, that 
the numerical relations which connect heat and energy in the 
case of friction hold good when energy and heat produce or 
destroy each other by any other means. 4. In the case of 
friction itself, energy is not transformed simply into heat, but 
partly into heat and partly into another kind of energy, which is 
involved in the expansion of the solids or liquids acted on. 5. 
No doubt the coincidence between the mechanical equivalent of 
heat, found by Dr. Joule from friction, and that by M. Favre 
from working a magnetic engine, seems very striking ; but (1) 
the value of Favre’s experiment disappears on examination. It 
was but a single experiment, either never repeated, or never 
repeated with the same results; in a very delicate experiment 
there was only the difference of 300 units out of 18,000; and 
even the permanent enlargement which always takes place in 
magnets which are in use might account for this; and (2) 
numerous and long-continued experiments by M. Soret show 
results entirely discordant with the single one of M. Favre, 6. It 
seems incredible, that with the imperfectly constructed engine 
used by Joule and Scoresby, they should at the very first trial 
have succeeded in utilising two-thirds of the magnetism evolved, 
or capable of being evolved, by their battery; and Dr. Joule 
now tells us that according to his latest calculations of the 
mechanical equivalence of heat they utilised six-sevenths of the 
power of the battery. The only conclusion we can arrive at is, 
that the real power of the battery, and therefore of a grain of zinc, 
must have been much greater than he calculated. 7. For consider 
the disadvantages under which the engine acted: (1) the tem- 
porary and permanent magnets were never nearer than } of an 
inch apart. Though Dr. Joule assures us this does not affect 
the Aower of the engine, it certainly produces a waste of zinc, as 
the near approach of the magnets creates counter-currents which 
check materially the consumption of zinc. (2) The copper wire 
was not tested for conductivity ; a subject little thought of at 
that time, and it is found that a very small impurity in copper wire 
will very, very largely diminish the power of an electro-magnet. 
(3) The iron was not tested for specific capacity for magnetism, 
yet this is a most important point which is eve now but little 
appreciated. It is found practically that, if two electro-magnets 
be made from the very same piece of iron, most carefully 
prepared, with the very same length of the same wire, 
without the slightest assignable cause, one will some- 
times have three times the power of the other. Hence 
Iconclude that the maximum energy capable of being 
evolved by a grain of zinc must be very much greater than 
that assigned to it by Dr. Joule. 7. Dr. Hopkinson’s argument, 
in his paper lately read to this society, virtually amounted to 
this—that a well-constructed magnetic engine will get no more 
duty from a grain of zinc than an ill-constructed one ; and conse- 
quently, I presume, that magnets might be weakened to any 
extent, and removed to ever so great a distance from one another, 
without necessarily affecting the efficiency of the engine. 
8. Dr. Hopkinson has in his criticism strangely substituted 
(a—2) for (6). In Joule and Scoresby’s paper, the consumption 
of zinc is expressed not by (a—é) but by (2) ; and consequently 

the duty of a grain of zinc not by = but by = 3; and when 
the magnets are stronger and approach nearer to each other, 
even if W be not increased, (4) is diminished. 9. My 
argument was this, that since the accepted theory of the 
mechanical equivalence of heat is that production of energy 
absorbs, and destruction of energy produces, a definite amount 
of heat, if we find cases, as those of elastic wires, and water 
below its maximum density, in which destruction of energy pro- 
ducescold, not heat, then the doctrine of the mechanical equivalence 
of heat cannot be true; we might with equal justice call ita mechani- 
cal equivalence of cold. It is no reply to say that such facts are 
simple deductions from the laws of thermodynamics. This would 
only show that the laws of thermodynamics are inconsistent with 
the doctrine of the mechanical equivalence of heat. 10. The 
argument from the fire syringe I withdraw, as inconclusive. But I 
think my case was sufficiently established without it. 11. Joule 
and Scoresby in their paper incorrectly assume that if 

the quantities of electricity in the current at different times 
be represented by (a) and (4), the heat varies as a? to /2. 
This is only true where the resistance is the same. In 
the case before us the working of the engine introduces 
a fresh elementin resistance. 12. Again by assuming that (a—/) 
represents diminution of quantity of the current, avd the dimi- 
nution in the zine consumed, ad the heat converted into useful 
work, they involve the supposition either that less zinc produced 
equal heat, or that heat was changed into useful work which was 
never produced at all, and therefore could not be absorbed. In 
fact, there was no froof that any heat was absorbed at all. 
3- Itis said that in electro-plating, electro-magnetic engines, 
worked by steam, are found more economical than batteries. 
This is in cases where a battery of many cells would be re- 
quired ; which is always wasteful, as a large number of equiva- 
lents of zinc must be consumed to deposit one equivalent of silver 
or other metal. 14. Besides, there is a far greater advantage in 
changing work into electricity, than electricity into work. In the 
former case all, or nearly all, the work is effective; in the latter, 
a very small portion of the electricity has hitherto been utilised. 
—Dr. Hopkinson said that most of Mr. Highton’s objections 
to the mechanical equivalent of heat appear to arise from a 
mistake as to what is meant by the term. ‘The nature of 
this mistake may be best seen in the case of a perfect heat 
engine, of which ¢, and 4, are the absolute temperatures of the 
source and refrigerator, Then from every unit of heat 
-  f—te 2 
leaving the source we obtain ae J units of work. Now 
1 
this a quantity variable with 4 and ¢,; it would be similar 
to most of Mr. Highton’s arguments to infer that from a 
given quantity of heat a variable quantity of work could be 
obtained. But, of course, the case really is that of the unit of 
: ee : , 5 
heat leaving the source, aa is lost in the refrigerator, whilst 
1 
4—% 

disappears as /eat and is converted into the work done, 
1 
and the principle of the equivalence of heat and work asserts 
that J is constant. It will be seen that this is the mistake 
Mr. Highton makes in his paper in the Journal of Science 
(end of article 6). He seems there to imagine it stated, 
that the work done is equivalent to the whole heat thrown 
into the gas, and he fails to perceive that a certain portion 
is used to raise the temperature of the air or turpentine. 
This will make my criticism of his paper in the Chemical 
News clearer, Mr. Highton argued against the mechanical 
equivalent, and what I pointed out was, that the chemical 
energy, which was converted into mechanical effect and not 
used to heat the wire, was proportional to a—é, that therefore, 
in order to prove that there was no mechanical equivalent 
y 
Mr. Highton must show WV is variable. Ido not assert that 
a-Oo 
a badly constructed engine will get as much heat from fuel as a 
gond one, but merely that the work done and the heat, which 
has disappeared as heat and been converted into work, are ina 
constant ratio. Now as regards Mr. Highton’s argument from 
the case of elastic wires—that the wire will be cooled when 
stretched follows from the two laws of thermodynamics, a proof 
may be seen in Tait’s Thermodynamics, p. 105. Mr. Highton 
replies, “ Quite true ; but this only shows that one of the laws of 
thermodynamics is inconsistent with the doctrine of the mechanical 
equivalence of heat.” Now the first law of thermodynamics asserts 
nothing else than that there isa mechanical equivalent, constant in 
all cases ; whilst the second law, as usually stated, involves the first 
law, and involves nothing else but Carnot’s axiom and the principle 
that in conduction heat flows from the hot to the cold body, both 
of which no one will doubt. Mr. Highton’s reply is very similar 
to stating that one of Kepler’s laws is inconsistent with the planets 
moving in ellipses. What Mr. Highton proposes as a paradox is 
then a necessary consequence of the principle he attacks. 
Though the doctrine of the mechanical equivalent of heat finds 
its firmest basis in the immortal experiments of Dr. Joule, the 
fact, that assuming it we can explain many phenomena, is a 
valuable supplementary proof. 
EDINBURGH 
Royal Physical Society, February 22.—Dr. Robert Brown, 
President, in the chair. ‘‘ On the Glacial Epoch,” by the Rev. 
LP. A. Brodie. The author of this paper proposed three questions— 
(1) Is he correct in supposing that the popularly received opinion 
with respect to the glacial epoch regards it as a period of com 
