376 SCIENCE PROGRESS 



Lindblad, B., On the Use of the Grating for Determining Spectral Type and 



Absolute Magnitude of the Stars, Astroph. Journ. 1919,49, 289. 

 Lundahl, C. F., On Some Properties of the Stars of Spectral Type F., Lund. 



Medd., Ser. II., No. 21, 1919. 

 Ludendorff, H., Zur Statistik der 8 Cephei Sterne, Ast. Nach. 1919, 209, Nos. 



5005-6. 

 Hagen, J. G., Das Blinksternproblem, ibid. 1919, 209, No. 4995. 

 Harvard Observatory, Proper Motions of Stars in the Zone -9°5o'to -i4°io', 



H.A., 1919, 81, No. 1. 

 Bailey, S. I., Variable Stars in the Cluster Messier 15, ibid. 1919, 78, No. 3. 

 Reynolds, J. H., The Distribution of Hydrogen and Nebulium in the Orion 



Nebula, M.N., R.A.S., 1919, 79, 561. 

 CANNON, J. B., Orbit of Spectroscopic Binary 32 # 2 Cygni, Pub. Dom. Obs. Canada, 



1919, 4, No. 10; Orbit of Boss 1082, ibid. No. 11 ; The Spectroscopic Binary 



Boss 1275, ibid. No. 12. 

 Harper, W. E., Orbits of the Spectroscopic Components of Boss 5173, ibid. 1919, 



4, No. 13. 

 Shapley, H. and M. B., Studies based on the Colours and Magnitudes in Stellar 



Cluster : (12) Remarks on the Arrangement of the Sidereal Universe, 



Astroph. Journ. 1919, 49, 289; (13) The Galactic Planes in 41 Globular 



Clusters ; (14) Further Remarks on the Structure of the Galactic System, 



Mt. Wilson Conts., 1919, Nos. 161-2. 

 Kester, F. E. and Alter, D., Spectral Series and the Grouping of the Asteroids, 



Astroph. Journ., 19 19, 50, 50. 



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

 University, Liverpool. 



The Source of Radio- Active Change. — As a result of the work 

 of several investigators during the last few years, it is now 

 coming to be generally recognised that all chemical reactions, 

 whether thermal or photo-chemical, are due to radiation. In 

 photo-chemical changes, as ordinarily examined, the tempera- 

 ture of the external source of light and therefore of the radia- 

 tion itself is considerably higher than that of the material 

 upon which the radiation acts. In ordinary or thermal 

 actions the origin and source of the chemical change is the 

 radiation present in the matter, and in equilibrium with it, 

 in virtue of the temperature of the matter. In ordinary 

 chemical changes we include such physical processes as change 

 of state. 



This wide significance of radiation for chemical and physical 

 changes has been extended still further by Perrin in a recent 

 paper (Annales de Physique [ix], 11, 19 19), who attempts to 

 find the origin of radio-active change in the existence of an 

 extremely short-wave type of radiation — " ultra-X " rays — 

 emanating from the earth and capable of penetrating matter 

 to a much greater extent than X-rays themselves. At first 



