Notices respecting New Books. 307 
could hitherto have passed unnoticed. Normally the effect 
is too small to be considered. ‘This force might, however, be 
successfully looked for with larger quantities of radium. 
According to Rutherford (Phil. Mag. May 1903, p. 588) the 
kinetic energy of each a-ray is 10-° ergs and their velocity 
2°5 x 10° ems. ; hence MV =8 x 10-® dynes, and this is the 
force for one ray projected. If 2x 10° rays per second are 
projected from each milligramme of radium having an activity 
one million times that of uranium, the reactionary force of 100 
milligrammes is 1°6 x 10-° dynes. This should be observable, 
but complications would arise from motions due to thermal 
convection-currents in the surrounding gas arising from the 
spontaneous evolution of heat in the radium. ‘The latter 
source of energy would, indeed, probably be sufficient to 
alone determine a rotation with suitably disposed vanes. 

XXXIV. Notices respecting New Books. 
Traité de Chimie Physique. Les Principes. Par Juan PERRIN, 
Chargé du Cours de Chimie Physique a la Faculté des Sciences de 
Paris. Paris: Gauthier-Villars. 1903. Pp. xxvi+299. 
OXE of the indications of the rapid strides which are being made 
in physical chemistry, and of the increasing interest which is 
being taken in this highly important but difficult branch of science, 
is the appearance within recent times of several important text- 
books on the subject. Of these, the book now under review must 
be regarded as one of the most important. 
Modern theories of chemical equilibrium are based on thermo- 
dynamical methods; and in the whole range of science there is 
probably no subject which presents greater difficulties to the 
beginner or is more full of pitfalls than thermodynamics. That: 
such is the case has been abundantly proved by the somewhat 
heated controversy regarding entropy carried on recently in a 
number of leading technical journals. Students of this subject will 
therefore feel grateful to M. Perrin for the fearless manner in 
which he handles real difficulties, and for the searching criticism 
which he brings to bear on slovenly or superficial methods of ex- 
position. He may, at times, be hypercritical and somewhat too 
fastidious, but the reader can only be the gainer by such scrupulous 
care in the handling of a difficult subject. 
The present volume is the introductory one of a comprehensive 
treatise, and a brief outline of its contents will give some idea of 
its scope. Chapter I. deals with the notion of generalized forces, 
Chapter II. with the factors of energy, Chapter III. with the 
principle of equivalence of the various forms of energy, Chapter IV. 
with the part played by the various factors of energy in producing 
changes in a system, Chapter V. with the principle of evolution, 
according to which the physical universe never returns to a 
previous state of existence, Chapter VI. with the characteristics of 
