RECENT ADVANCES IN SCIENCE 25 



quency intervals between the doublets increases by a constant 

 amount as we pass from each one to the one of next higher 

 order. A paper in the January number of the Phys. Rev. 

 discusses the resonance radiation of sodium vapour when 

 excited by the light from one of the D lines of incandescent 

 sodium vapour. If for instance, D 2 light is used, under certain 

 conditions Di light (of weak intensity) as well as D 2 light 

 appears in the resonance radiation, and from a study of the 

 conditions in this case (as well as of the changes introduced 

 by admitting a little hydrogen into the resonating vapour) 

 Wood concludes that the transmission of energy from the 

 D 2 emission centres of the sodium atom (excited by the D 2 

 light) to the Di emission centres is in some way the result 

 of molecular collisions either of sodium with sodium (when 

 it is dense enough or hot enough) or of sodium with hydrogen. 



PHYSICAL CHEMISTRY. By Prof. W. C. McC. Lewis, M.A., D.Sc, 

 University, Liverpool. 



Colloids and Chemical Industry. — The events of the past three 

 and a half years have given to the whole subject of chemistry 

 a prominence which under normal conditions it would probably 

 not have received in this country for fifty years. At long last 

 this country has awakened to a partial appreciation of the funda- 

 mental role which chemical operations play in the economic 

 life of a nation. This awakening has made itself felt through- 

 out the entire range of chemistry pure and applied, with the 

 result that matters formerly regarded as relatively unimportant 

 (even in the opinion of trained chemists themselves) have now 

 taken on a new interest and significance. 



One could scarcely find a more striking illustration than 

 that afforded by the subject of colloid chemistry, itself a 

 branch of physical chemistry. Although the pioneer work in 

 this field was first carried out many years ago by Graham, 

 the subject as a whole has remained the Cinderella of physico- 

 chemical science, attracting indeed a limited amount of atten- 

 tion in certain specialised directions, but remaining practically 

 a terra incognita so far as the majority of chemists, academic 

 as well as technical, were concerned. The vast possibilities 

 which underlie this branch of chemistry are as yet by no 

 means fully grasped. It has been recognised, however, that 



