Mixed Colouring Blatters with the Sjpecfrum Microscope. 133 



formed so readily that it is sometimes almost impossible to obtain 

 an aqueous solution of the mixed colours free from it. 



All these substances belong to a single natural group, dis- 

 tinguished by certain marked characters, and since the mixture of 

 several of them has been named Fhycoerijthrin, it may be well to 

 call it the Phycoerythrin group. It differs entirely from the Ery- 

 throjyhyll group, described in my paper " On the Colouring Matter 

 of Leaves," in the fact that the position of the bands is very little, 

 if at all, changed by the addition of weak acids or alkalies ; but when 

 they are stronger, actual decomposition occurs, some of them being 

 more easily changed by acids and some by alkalies. The difference 

 in the spectra shown in the woodcut cannot therefore be caused by 

 any variation in the acid reaction of the juice of the plants, as is 

 the case in the colouring matter of the petals of some flowers. 

 The possibility of this must always be borne in mind in applying 

 such a method of comparison, and the effect of various reagents 

 must always be ascertained before the actual identity of the colour- 

 ing matters can be inferred from the agreement in the bands in 

 only one particular condition. Much may be learned by acting 

 directly on the plants themselves. 



The consideration of the various spectra described above seems 

 to lead to the following conclusions : — 



1. When a spectrum shows two absorption-bands, like No. 5, 

 they should not be considered due to one single substance until 

 satisfactory evidence of the fact has been obtained. The solution 

 should be allowed to undergo slow decomposition, and be repeatedly 

 examined, in order to ascertain whether both bands disappear in the 

 same proportion, and also the action of various reagents observed, 

 in order to learn whether one band can be permanently removejd 

 without the other, making of course due allowance for any change 

 that may depend merely on an acid or alkaline reaction. 



2. When more than two bands are seen in the spectrum, as in 

 No. 3, and they are not at nearly equal intervals, the compound 

 nature of the substance may be considered so probable, that further 

 examination should certainly not be neglected. 



3. When there is broad shading about a narrow absorption - 

 band, as in No. 2, it is important to ascertain whether or not it is 

 due to the same substance. There are certainly many cases in 

 which I have always concluded that both are due to the same, but 

 examples like this evidently show that such an opinion ought not 

 to be formed until after further examination. 



The occurrence of so many assQciated colouring matters, as in 

 Algae, may be rare. It must not be supposed that I imagine that 

 whenever there are two or more absorption-bands they are due to 

 two or more independent substances. As an example of what I 

 look upon as satisfactory proof of the contrary, I will describe some 



