288 THE CHEMISTRY OF THE SUN. [CHAP. 



that he accepts the view of molecular simplification that I 

 advocate : 



" According to [one] view, liquid and solid bodies give generally 

 continuous spectra, not because they are liquid or solid, but because in 

 these states the molecules have a more complicated structure than 

 in the gaseous state. Experiment has to decide between the two 

 theories, the theory of molecular disturbance and the theory of 

 molecular structure. I think the facts are decidedly in favour of 

 the latter theory. Mr. Lockyer's investigations have shown that 

 most bodies give us a continuous spectrum, as a gas, before they 

 condense, and many at a considerable temperature above the boiling 

 point. Mr. Lockyer has rightly drawn the conclusion from this 

 fact, that the atomic aggregation of the molecules is the cause of 

 the different orders of spectrum. If we observe the changes in a 

 spectrum which gradually take place on heating or cooling a vapour, 

 we find that the continuous spectrum is produced, not by a 

 widening of the bands, but by a direct replacement, which is some- 

 times sudden and sometimes gradual, and which leaves no doubt in 

 the observer's mind that he has to deal with two vibrating systems, 

 and not simply with a disturbed one ', I do not, of course, mean to say 

 that the impacts of other molecules have no observable influence. 

 If the hydrogen lines widen through increased pressure, it is very 

 likely that the alteration is produced by impacts ; but the change 

 from a line-spectrum to a continuous-spectrum, as a rule, is quite 

 different from the change which takes place with hydrogen. Accord- 

 ing to the theory of molecular aggregation, it seems quite possible 

 that a liquid should give the same spectrum as its vapour, and this 

 indeed seems to be true in some cases." 1 



In a subsequent note Dr. Schuster adds 



" That the discontinuous spectra of different orders (line and band 

 spectra) are due to different molecular combinations I consider to be 

 pretty well established ; and analogy has led me (and Mr. Lockyer 

 before me) to explain the continuous spectra by the same cause ; for 

 the change of the continuous spectrum to the line or band spectrum 

 takes place in exactly the same way as the change of spectra of 

 different orders into each other." 2 



1 Phil. Trans. Part I. 1879, p. 38. * Loc. cit. p. 3. 



