194 SCIENCE PROGRESS 



192 1, 28, 115) and Weigert and Pohle {Kolloid Zeit., 192 1, 88, 

 153) have made important contributions to the study of the 

 changes in disperse systems under the influence of coloured 

 light. It is well known that a silver chloride gelatine film, which 

 has been previously exposed to light and thoroughly washed with 

 water, changes in colour when exposed to coloured light. The 

 colour of the film is changed to a more reddish, greenish, or 

 bluish tint after exposure to red, green, or blue light respectively. 

 The extinction (E = log lo/I) of the film for a particular wave- 

 length of light is decreased by exposure to that wave-length and 

 increased by exposure to any other wave-length. Plane polarised 

 light so modifies the silver gelatine film that the extinction 

 to coloured plane polarised light varies with the direction 

 of the plane of polarisation. The film, which can thus be said 

 to become dichroic, at the same time becomes double-refracting. 

 Both efi"ects are produced best by light of long wave-length. 

 A chemical explanation of the phenomenon does not appear to 

 be possible, since the number and amount of the silver particles 

 are unchanged by illumination. An examination of a liquid 

 emulsion by the ultramicroscope showed that the ultramicrons 

 had little influence on the phenomenon. The coloured dichroic 

 ultramicrons observed by Siedentopf in silver chloride could be 

 produced by exposure of the colourless ultramicrons to coloured 

 light. The colour of the particles was independent of the 

 exciting light, and contrary to the production of dichroic and 

 double-refracting films light of short wave-length was most 

 effective. Weigert comes to the conclusion that large numbers 

 of amicrons, which cannot be resolved by the ultramicroscope, 

 are in some manner responsible for the colour changes which 

 take place in the silver gelatine film. 



Friend {Trans. Chem. Soc, 1921, 119, 932) has continued 

 his work on the corrosion of iron. Iron corrodes less rapidly 

 in a very rapid stream of water than in a slow stream. The 

 author explains this behaviour on the assumption that ferric 

 hydroxide catalyses the corrosion of iron, and that this substance 

 would be removed by a rapid stream of water. He finds that 

 the influence of dissolved electrolyte and protection of ferric 

 hydroxide by colloids is in agreement with the theory. 



Hahn (Ber., 192 1, 64, 1131) claims to have discovered a 

 new radio-active substance in ordinary uranium salt with the 

 chemical properties of proto-actinium. It evolves /3 rays and 

 possesses a half life period of 6-7 hours. 



Aston {Phil, Mag., 1921, 42, 141) shows that iodine is a 

 simple element with an atomic weight of 127. This is contrary 

 to the theories of Broek and others. Kohlweiler has deduced 

 that five iodines occur, and claims to have partially separated 

 these by diffusion. 



