26 REPORT — 1891. 



reflected, refracted, polarised, and made to interfere. Thus, it 

 has been proved, both by theory and experiment, that hght is 

 simply an electric phenomenon. 



During the last few years an enormous amount of valuable 

 work has been done in the department of chemical physics, of 

 which, perhaps, the most important is that which began with 

 Raoult's study of the freezing-points and vapour tensions of 

 dilute solutions, and led to the discovery by Raoult himself of 

 new methods for the determination of molecular weights, to the 

 formulation by van't Hoff of a new theory of the nature of 

 solutions based on the laws of osmotic pressure, to a great 

 extension by Arrhenius of the theory of dissociation of electro- 

 lytes (first conceived by Williamson and Clausius), and to 

 the discovery and application by Ostwald of the method 

 of deducing the coefficient of chemical affinity from measure- 

 ments of electric conductivity. 



As I understand that this department of modern research 

 is to be treated at this meeting by the esteemed President of 

 the Chemical Section, Dr. Masson, who has made a special 

 study of the subject, I will not run the risk of displaying my 

 ignorance by saying anything more about it. 



Considerable advances have been made in the study of the 

 behaviour of definite liquid compounds with regard to changes 

 of temperature, pressure, and volume in the neighbourhood of 

 their critical points, particularly by Professors Ramsay and 

 Young. Some of the theoretical conclusions of Van der Waals 

 have been thus put to the test of experiment, and shown to be 

 erroneous, while others are apparently confirmed. 



In this connection may be i:ioticed our increasing knowledge 

 of the properties in the liquid state of those substances which 

 we ordinarily know as gases. It is not so many years since 

 Andrews's classical research on the continuity of the liquid 

 and gaseous states first bore practical fruit in the liquefaction 

 by Pictet and by Cailletet in 1877 of the old permanent 

 gases. 



Since then methods have been so much improved that 

 experiments caii be carried on at as low a temperature as 

 — 210°C. To Wroblevsky, Olszweski, and others we owe quite 

 an intimate knowledge of liquid oxygen, nitrogen, marsh-gas, 

 and nitric oxide ; but it appears to be doubtful whether 

 hydrogen does not maintain a unique position as an uncon- 

 densed, though surely not uncondensable, gas. Experiments 

 seem to show that at these exceptionally low temperatures 

 chemical action is at a standstill, and it is hardly to be 

 wondered at when such proximity to the absolute zero is 

 attained. 



In few branches of physical science has so great an advance 

 of late been made as in the methods for measuring extremely 



