CHEMISTRY. 293 



the molecular formulas being AS4O6 and Sb406 as in the gaseous state. There 

 has also been investigated the structure of KHF2 (by Dr. Bozorth) and that 

 of the hexagonal mineral molybdenite (by Dr. Dickinson and Mr. Linus 

 Pauling). Mr. Pauling has studied single crystals of the intermetallic com- 

 pound Mg2Sn, and has found its structure to be like that of CaF2; this being 

 apparently the first complete determination of the structure of an intermetallic 

 compound. The substance NaCd2 was also investigated, but it was found to 

 be surprisingly complicated. 



A study by Dr. Dickinson of some radial streaks and anomalous spots 

 observed in Laue photographs has led to the hypothesis that they are 

 due to the existence in the crystal of a great number of small crystals indi- 

 vidually perfect but with their axes slightly inclined to those of the main 

 crystal, the number of individuals having a given inclination being smaller 

 the greater the inclination. 



5. Rates of Chemical Reactions. 



Fundamental theoretical studies by Dr. R. C. Tolman, and experimental 

 researches under his direction, have been initiated in this laboratory in an 

 attempt to contribute to the principles underlying the rate of chemical 

 reactions. These principles have long been one of the most important and 

 baffling problems of theoretical chemistry. The laws of thermodynamics 

 provide a theoretical basis for predicting what chemical reactions can take 

 place and how far they will proceed before equilibrium is attained. Thermo- 

 dynamics, however, is powerless to determine the rate at which the thermo- 

 dynamically possible reactions will actually proceed. The final solution of 

 the problems of chemical velocity will therefore be of extraordinary impor- 

 tance, since it will enable the chemist to predict not only the possible reac- 

 tions in a given mixture of substances, but also the actual reactions which 

 really do take place. 



The effect of temperature on the rate of reaction, and the factors which 

 determine the specific rate of reaction at any given temperature are questions 

 which have not yet received adequate theoretical explanation. The most 

 promising suggestion for their solution has been the radiation hypothesis of 

 Perrin and W. M. C. Lewis, which attributes the activation of chemical 

 molecules to the absorption of radiant energy from the black-body radiation 

 corresponding to the temperature of the reacting mixture. Nevertheless, it 

 has been shown definitely by the work of Daniels and Johnston on the de- 

 composition of nitrogen pentoxide that light of the predicted frequency will 

 not cause decomposition. 



This failure, however, need not force us to complete abandonment of the 

 radiation hypothesis. It was pointed out by Tolman in 1920 that the theory 

 of Perrin and Lewis was seriously handicapped by the assumption that 

 activation was produced by a single frequency, while it is known experi- 

 mentally that photochemical reactions can often be produced by a wide range 

 of frequencies; and he developed a more generalized theory of reaction. 



Work in this laboratory for testing this theory has been undertaken with 

 the help of Mr. Oliver R. Wulf ; the reaction between bromine and cinnamic 

 acid in carbon-tetrachloride solution being first investigated. The thermal 

 reaction-rate has already been determined at 25° and 45°; and the reaction 

 has been shown to proceed with a measurable photochemical rate in light 

 of reasonable intensity. Apparatus is now being installed for the production 



