1875.] Physics. 527 
occurs at a dull red heat. On the other hand, the cooling of the wire may be 
represented as in Fig. 7. Here there are two points, dd’, at different intervals 
of time in cooling, where, nevertheless, the temperature of the wire is the 
same. The curves, therefore, are not symmetrical. Of course the continued 
accession of heat in Fig. 6 would mask any small fall in temperature if such 
occurred. In both cases the anomalous temperature is seen only as the result 
of a differential action. 
After a remark by Professor Clerk Maxwell on the analogy of these phe- 
nomena with Mr. Spencer’s statement on the propagation of nerve force 
already referred to,— 
Mr HERBERT SPENCER replied—I rather think that Mr. Barrett’s experi- 
ments confirm my hypothesis. When the nerve impulse has been expended, 
I infer that the nerves are reinforced from the neighbouring tissues. They 
fall in temperature, and instantly absorb heat from adjacent tissues. As a 
verification of this inference, I may mention we have evidence that the velocity 
of nervous transmission is variable according to the temperature of the body— 
the current being swifter in summer than in winter, which corresponds with 
the fact that the ‘‘ personal equation” is a variable element in different indi- 
viduals, and may be presumed to vary in consequence of their slight difference 
of temperature. 
Professor MAxwELL—I attribute to Mr. Spencer the view that the nerve 
recovers its power of acting by absorbing heat. The people who read such a 
statement might attribute to him the notion that the nerve a¢ts as a heat- 
engine. It may be that when the nerve is raised to a higher temperature, 
some chemical action may be able to take place which, though it is a lowering 
of the available energy of the system on the whole, may make it more available 
for giving another impulse—another wave of nerve force—but at the same 
time, after all is done, the nerve is worse off than before, so that it has to 
Teceive from the blood something to bring it up to its former state. The two 
sides of the interpretation are these: We all admit that nerves won’t go on 
without food, but it may be possible that some action, depending upon the 
absorption of heat from the surrounding tissues, may make the nerve more 
instantaneously available. But those who believe in thermo-dynamics, and 
the dissipation of energy, cannot admit that any purely material system can 
convert heat into work when the system and its whole environment are, at 
the commencement of the operation, at the same temperature. There isa 
statement in Sir W. Grove’s address on continuity, in which he mentions that 
Berthelot had discovered that a certain salt could undergo chemical change in 
which its energy was increased by absorption of heat from the surrounding 
medium, at a temperature no higher than its own. That statement has not 
been attacked; but, though it supports Mr. Spencer’s view, it cannot be under- 
stood literally by anyone who believes in Carnot’s law, which expresses under 
what conditions heat can do work in passing from one thing to another. 
Mr. HERBERT SPENCER—I by no means assume that there is any genesis 
of force made possible in a nerve by any such absorption of heat. Although 
not very familiar with thermo-dynamics, I am sufficiently so to be aware that 
there is no possibility of such an evolution of force in the nerve except at the 
expense of the body at large. Iam merely endeavouring to show that we have 
in the heat of the tissues surrounding the nerve a reserve of force ever present, 
which may be transformed into the force that traverses a nerve simply by the 
instrumentality of the change in the isomeric forms of the molecules of 
nerve; which change is a change of a kind permitting them, each time they 
absorb heat, to assume a less stable form, and therefore again to fall into the 
more stable form. It is a perfectly reasonable hypothesis that a nerve may 
be composed of material which, like all the other forms of protein, admits of 
extremely easy change from one isomeric state to another. If it is supposed 
that by the absorption of a small amount of heat, the molecules of the 
nerve assume such a relation, that by a slight disturbance they will again 
resume their more stable state, and send a cumulative wave of disturbance—like 
Professor Maxwell’s line of bricks:—I say it is a feasible assumption that 
when they have done that, they have lost heat to the extent that they have 
