290 Dr. E. Budde on the Action of Light 



as I show above, be considerably larger than the* observed amount 

 (50"'l) if the crust were rigid, it would be reduced to the ob- 

 served quantity owing to its yielding in some measure to the 

 disturbing forces. This, however, is answered by Sir William 

 Thomson's investigation on the Rigidity of the Earth . He shows 

 that if the material of the earth yielded as little as even a globe 

 of steel or glass would under the action of the disturbing forces 

 of the sun and moon, the effect on both the tides and the preces- 

 sion would be very perceptible, and both would be less than for 

 a rigid earth. This we know is not the case. I think, therefore, 

 that geologists must submit to the verdict that the crust of the 

 earth is very thick, if not solid to the centre, and must be con- 

 tent with the idea that there are local seas of lava in the crust 

 itself to account for volcanic phenomena. 



John H. Pratt. 

 Calcutta, August 5, 1871. 



XXXI. On the Action of Light on Chlorine and Bromine. 

 By Dr. E. Budde, of the University of Bonn*. 



IN the course of an investigation on the combustion of explo- 

 sive gaseous mixtures and on catalytic action in which I 

 am engaged, I frequently had to apply the well-known propo- 

 sition of Favre and Silbermann and Clausius, according to 

 which the molecules of most elementary gases consist of two 

 atoms j and in the experimental prosecution of deductions drawn 

 from this hypothesis I have arrived at some rather remarkable 

 results with regard to chlorine and bromine. Assuming the 

 hypothesis to be true (and certainly it is as probable as Avoga- 

 dro's theorem and the equality of specific heats of gases for equal 

 volumes), it naturally leads to the conclusion that the so-called 

 combination of two elementary gases must in general be pre- 

 ceded by a splitting up of their molecules into isolated atoms, 

 and that consequently such a combination will be promoted by 

 any influence which induces a separation from each other of 

 equal atoms without hindering the combination of the unequal 

 ones. 



Now it is known that in chlorine, through insolation, there is 

 induced a higher degree of chemical activity. This fact might be 

 accounted for in two ways — (1) by assuming that light increases 

 the attraction between the atoms CI of chlorine and the atoms k 

 of the respective other body, and (2) by assuming that light 

 tends to resolve, or actually does resolve, the chlorine molecule 

 into its constituent atoms. Of these two hypotheses the former 

 does not appear very probable — the less so, as, for instance, in the 



* Communicated bv the Author. 



