1876.] 
Notices of Books . 
109 
in and were laid at the head of the table. The president took up 
one, pronounced its generic and specific name, and handed it to 
his right-hand neighbour. He in turn examined it, if he thought 
needful, and passed it on, repeating the name, which, by the 
time the plant had travelled round the table was often strangely 
travestied. In this manner numbers of men learn by rote the 
names of all ordinary species in the British flora, but if they 
were asked the reason for referring any particular plant to some 
given genus they would be unable to reply. Precisely the same 
practice exists as regards insedts. They learn that such and 
such British butterflies bear the generic name Vanessa, but if 
shown an exotic Vanessa they would be at a loss where to place 
it. Surely no branch of Natural History should be left to rest 
upon such a foundation. 
Theory of Heat. By J. Clerk Maxwell, F.R.S., &c. Fourth 
Edition. London : Longmans, Green, and Co. 
This book, though it may be pronounced indispensable for every 
student of physics, is not, in the ordinary sense of the word, a 
manual of the science of heat. Its object, as declared in the 
preface, is “ to exhibit the scientific connection of the various 
steps by which our knowledge of the phenomena of heat has been 
extended.” For an account of many important experiments on 
the effects of heat, which could not have been introduced without 
extending the work to an inconvenient bulk, the reader is referred 
to other well-known publications. 
The most profound interest attaches to the last section in 
which the distinguished author treats on the “ nature and origin 
of molecules.” The molecules of the same substance, he declares, 
are all exactly alike, but different from those of other substances. 
There is no regular gradation in their mass from that of hydrogen 
up to that of bismuth, but they fall into certain classes or species. 
Unlike the case of plants and animals, it is not possible, Dr. 
Maxwell maintains, to account for the state of molecules by 
natural selection. Each individual is permanent ; there is neither 
generation or decay, nor even any difference between the indivi- 
duals of each kind. Hence the doctrine of evolution is here 
inapplicable. Hence to account for the equality of molecules in 
magnitude and in their natural periods of vibration we must 
either, as the author once expressed this view, declare that they 
bear the stamp of manufactured articles, or consider them as the 
primordial materials of the cosmos, the bricks with which its 
architects — conscious or unconscious, mediate or immediate — 
have had to work. Neither of these hypotheses is really satis- 
factory. To pronounce them primordial is something like the 
explanation a savage might give of the origin of a mountain. To 
declare them manufadtured is merely to elicit further question as 
to their raw material. 
