September i6, 1920] 



NATURE 



73 



working hypothesis, but the experiments of Sir 

 Dugald Clerk on the variation of specific heat, 

 and on the composition of the mixture shortly 

 after the attainment of maximum pressure, appear 

 to show that the question is not quite so simple. 

 The later development of Hopkinson's own ex- 

 periments on radiation in gaseous explosions by 

 W. T. David indicates other factors which have 

 to be considered in framing a complete theory, 

 and may have an important influence on the 

 design of engines for large units of power or for 

 high speeds. The completeness or otherwise of 

 combustion at the moment of maximum pressure 

 may fairly be regarded as still an open question 

 and a fit subject for further experiment. 



In developing the general thermodynamic rela- 

 tions between the various properties of a sub- 

 stance, the chief difficulty is to make a selection 

 unong the many possible permutations. It need 

 scarcely be said that the author makes a very 

 judicious selection for the purpose in view, and 

 develops chiefly those relations of the energy, 

 ■ ntropy, and total heat, and of their coefficients, 

 ! !ie specific heats and the cooling-effects, which 

 ire required in applying the principles of thermo- 

 dynamics to the correlation of the properties of 

 ilie working fluids employed in heat-engines. In 

 the next chapter he proceeds to apply the general 

 relations to particular fluids, defined by well- 

 known forms of characteristic equation, showing 

 liow the equation selected determines the neces- 

 sary relations between the coetFicients. In this 

 connection the author discusses the general pro- 

 perties of gases and vapours, as illustrated by 

 Amagat's experiments at high pressures, and Van 

 der VVaals's equation for the critical state. But 

 most of the numerical illustrations are drawn from 

 the properties of steam at moderate pressures, as 

 being the most important for engineering pur- 

 poses and the most accurately determined by ex- 

 periment. He shows how the formula: employed 

 secure exact thermodynamic consistency between 

 the various properties, but he refrains from dis- 

 cussing the experimental evidence with regard to 

 the particular values selected for the constants, 

 because this is a separate question beyond the 

 scope of pure thermodynamics. 



.^t the same time the book is not without some 

 bearing on physical experiment, because no 

 modern experimentalist can afford to represent his 

 •;, results by purely empirical formula; without in- 

 quiring how far they arc consistent with other 

 measurable properties of the substance investi- 

 gated. This requires a knowledge of the thcrmo- 

 dynamical relations, and the ability to apply them, 

 which this book is intended to teach. Thus one 

 of the practical uses of the thermodynamic rela- 

 NO. 2655, VOL. 106] 



tions is to find the form and slope of lines on 

 diagrams, such as the MoUier diagram, which are 

 becoming so popular among engineers. MoUier 

 had great difficulty in constructing his diagram 

 for carbonic acid near the critical point, owing to 

 the scarcity and uncertainty of the experimental 

 data. Most of the missing data have since been 

 supplied by the experiments of Prof. Frewen 

 Jenkin and Mr. D. R. Pye, who made great use 

 of thermodynamic relations in constructing their 

 diagram, and incidentally showed that some of 

 MoUier's lines were of an impossible shape. They 

 had some difficulty, however, in fixing the form 

 of the saturation line for the liquid, which ap- 

 peared to show a point of inflexion a few degrees 

 below the critical point. The author suggests that 

 the point of inflexion should be at the critical 

 point itself. This is in some ways an attractive 

 suggestion, but the proof does not seem to be 

 quite conclusive, and would be difficult to recon- 

 cile with the orthodox view of the critical state, 

 according to which the critical isothermal itself 

 possesses this property. It is possible that the 

 singularity at the critical point may be of a nature 

 different from that- commonly assumed. It 

 would be difficult to decide such a point by experi- 

 ment, but it is just in such cases where experi- 

 ment fails that thermodynamics is most useful. 



H. L. C. 



Forensic Medicine. 

 .1 Handbook of Medical Jurisprudence and Toxi- 

 cology for the Use of Students and Practi- 

 tioners. By Dr. VV. A. Brend. Third edition, 

 revised. (Griffin's Medical Pocket-book Series.) 

 Pp. xiii + 317. (London: Charles Griffin and 

 Co., Ltd., 1919.) Price los. 6d. net. 



THIS edition has been revised and enlarged. 

 The chapter on the legal relationships of 

 insanity and other abnormal states of mind, which 

 has been entirely rewritten, gives a very good 

 account in a brief and concise form of a branch 

 of forensic medicine which usually presents diffi- 

 culties to the medical student and practitioner. 

 Chap, xvii., dealing with "Medical Privileges and 

 Obligations," is excellent, and gives in a collected 

 form the duties and responsibilities which may 

 devolve upon a medical practitioner as a result of 

 his registration by the General Medical Council. 

 This subject has been much neglected in most of 

 the existing text-lx>oks on forensic medicine, and 

 the present edition of this work supplies a much- 

 needed want by providing valuable advice on the 

 line of conduct to he adopted by medical men in 

 the many difficult situations which are likely to 

 arise in medical practice. 



