Mr. H. C. Sorby on 



[JuilC 19 } 



is due to the luminous rays, which act with variable intensity indifferent 

 parts of the spectrum, according to the nature of the substance. On the 

 contrary, my method of photochemical analysis consists in using either 

 the whole or particular rays of the sunlight passing through coloured 

 glasses as reagents to decompose some of the constituents of a mixed 

 solution, and leave others in a state of approximate purity. In em- 

 ploying light for such a practical purpose, it would be almost impossible 

 to use a spectrum. Without this method the study of the colouring- 

 matters of plants could not be carried out in a successful manner. I 

 have sometimes studied a substance for several weeks before I was able 

 to prove that it was a mixture by ordinary chemical methods, whereas I 

 afterwards found that this could be easily proved by simply exposing the 

 solution to the sun. I, however, now give only such examples as have 

 an important bearing on the subject before me. For instance, in the 

 case of some Alc/ce it is impossible to separate the xanthophyll from the 

 orange xanthophyll, and the spectrum of the mixed solution in bisulphide 

 of carbon gives absorption-bands in an intermediate position, not 

 agreeing with those characteristic of either one or the other. It might 

 thus be imagined that these Algce differed entirely from allied species, in 

 containing a single and peculiar kind of xanthophyll instead of the usual 

 two, thus making an apparent complete break in what is really perfect 

 continuity ; but by exposing the solution to the open sun the orange 

 xanthophyll is so much more rapidly destroyed than the xanthophyll, 

 that in a while the absorption-bands of the latter are seen in their 

 normal position ; and knowing this fact, and comparing the spectrum 

 with that of the original, it is easy to see that at first there must have 

 been a mixture of the two kinds. Of course in all such experiments 

 care must be used to prevent overexposure, and the specimens should be 

 examined from time to time. In most cases it is also extremely difficult 

 to separate yellow xanthophyll from lichnoxanthine ; but by exposure to 

 the sun this xanthophyll is soon decomposed, its absorption-bands dis- 

 appear, and the uniform absorption of the lichnoxanthine remains. 

 Solutions which originally give very different spectra can thus be proved 

 to be merely variable mixtures of two or more substances, each well known 

 in a pure state. There are, however, cases in. which both constituents 

 would be destroyed by exposure to white light ; but one is changed 

 chiefly by one kind of rays, and the other by another. For example, 

 when the mixed solution in bisulphide of carbon of phycoxanthine and 

 orange lichnoxanthine, obtained from certain Algce and lichens, is exposed 

 to the open sun, the phycoxanthine is soon destroyed, but at the same 

 time some of the orange Hchnoxanthine, and it is almost impossible to 

 cease exposing at the proper point ; when kept under deep green glass, 

 however, it is easy to decompose nearly all the phycoxanthine without 

 materially diminishing the amount of the orange lichnoxanthine. Other 



