294 CARNEGIE INSTITUTION OF WASHINGTON. 



of light of known frequencies, and for the measurement of light intensities 

 for use with this reaction. 



With the help of Mr. Ernest C. White work is also in progress on the initial 

 rate of decomposition of nitrogen pentoxide. This is a matter of interest, 

 since previous work has shown that, though the reaction is of the first order 

 after it is once under way, the rate is very much slower in the absence of the 

 decomposition-product nitrogen dioxide. The work has necessitated the 

 design and construction of an elaborate all-glass colorimeter for measuring 

 small concentrations of nitrogen dioxide. The apparatus has been completed 

 and one preliminary run carried out. 



A theoretical investigation of the temperature-coefficient of photochem- 

 ical reaction rates has been published by Dr. Tolman. By the use of the 

 methods and concepts of the quantum theory, a new and more illuminating 

 derivation has been given of the same equation for temperature-coefficient 

 as was originally derived on the basis of classical statistical mechanics. 

 Existing data on temperature-coefficients have been successfully interpreted 

 with the help of this equation. An equation for the change in temperature- 

 coefficient with frequency was also derived, and successfully used in inter- 

 preting existing data; and the causes for apparent deviations from Einstein's 

 hypothesis of photochemical equivalence were briefly discussed. 



6. Theoretical Thermodynamics. 



A theoretical study has been carried out by Professor R. C. Tolman and 

 Mr. R. M. Badger on the entropy of diatomic gases and rotational heat- 

 capacity. This has appeared in the Journal of the American Chemical 

 Society. A direct mathematical method has been developed for determining 

 the theoretical value of the constant in the equation for the entropy of a dia- 

 tomic gas composed of molecules of the rigid dumb-bell model. Six different 

 theories which have been proposed, all based on the first form of the quantum 

 theory but involving different assumptions as to a priori probabilities and as 

 to the lowest possible azimuthal quantum number, have been used to calcu- 

 late the theoretical value of the constant, and the results have been compared 

 with existing experimental data. 



A theoretical study on rotational heat-capacity and half quantum numbers 

 has also been made by R. C. Tolman. It is shown that the low-temperature 

 data on the specific heat of hydrogen are not out of accord with the idea that 

 the lowest possible azimuthal quantum number is one-half. 



Richards, Theodore W., Harvard University, Cambridge, Massachusetts. 

 Continuation of exact investigation of at07nic weights and other physico- 

 chemical properties of elements and of simple compounds. (For previous 

 reports see Year Books Nos. 2-21.) 



The following investigations have been in progress during the academic 

 year, some of them continuations of work begun a year or more ago. 



1. The Atomic Weight op Radioactive Lead. 

 Dr. Paul Putzeys, Belgium Research Fellow, investigated the atomic 

 weight of radioactive lead obtained from the very interesting and important 

 deposits of the Congo. He proved conclusively that lead obtained from these 

 minerals consists largely of the uranium isotope of the metal, finding 206.2 

 as the atomic weight. 



