PHYSICAL CHEMISTRY 37i 



tion with respect to mixtures of electrolytes in solution. In 

 discussing this question the authors introduce a new term, 

 viz. the ionic strength ^ = sum of the stoichometrical molality 

 of each ion X the square of its valency and the whole divided 

 by two. The new general principle is that " in dilute solution 

 the thermodynamic degree of dissociation of a given strong 

 electrolyte is the same in all solutions of the same ionic 

 strength," and is independent of the particular character of the 

 other strong electrolytes present. Very satisfactory calcula- 

 tions of the solubility of salts in solutions of mixed electrolytes 

 can be made on these assumptions. 



Lewis points out that, while the results are in agreement 

 with the theory of complete dissociation, no definite proof of 

 this theory is yet forthcoming. 



ORGANIC CHEMISTKY. By O. L. Brady, D.Sc, F.I.C, University 

 College, London. 



Synthesis of Formaldehyde and Carbohydrates from Carbon 

 Dioxide and Water. — An important contribution towards the 

 elucidation of the problem of the synthesis of carbohydrates by 

 plants has been made by Baly, Heilbron and Barker {Trans. 

 Chem. Soc, 1921, 119, 1025), who have shown that Moore and 

 Webster's failure to detect formaldehyde in a saturated solution 

 of carbon dioxide in water exposed in quartz vessels to sunlight, 

 or to the light of a quartz-mercury lamp, was due to photo- 

 chemical polymerisation of the formaldehyde, as produced, to 

 reducing sugars. They find that if a stream of carbon dioxide 

 is passed through the liquid, some of the formaldehyde pro- 

 duced escapes polymerisation by being removed to the back 

 of the vessel, where it is protected from the light, and in these 

 circumstances it can be detected at the end of the experiment. 



It was found that the rays which brought about polymerisa- 

 tion were of longer wave length (X = 290 fifj,) than those required 

 for the synthesis of formaldehyde (\ = 200 fi/n), and that the 

 addition of a compound which ( i ) does not interfere in a chemical 

 sense, (2) selectively absorbs the specific rays which bring about 

 polymerisation, and (3) is transparent to the very short wave- 

 lengths, resulted in good yields of formaldehyde being produced 

 by the action of ultra-violet light on a solution of carbon 

 dioxide in water. Paraldehyde and sodium phenoxide both 

 fulfil these conditions, as also do the metallic salts, such as 

 ferric chloride or uranyl nitrate, which Moore and Webster 

 describe as catalysts for the synthesis of formaldehyde, but 

 which are apparently protective agents for this substance. 



A further difficulty, however, arises in that plants grow 

 and synthesise carbohydrates under glass which completely 



