448 Mr. H. C. Sorby on Analysis of Animal and Vegetable [Apr. 11^ 



16. Groups A, B, and C. 



Colours which are thus altered when the solution is ammoniacal, consti- 

 tute my group A. Frequently, however, sulphite of soda does not remove 

 the detached absorption when excess of ammonia is present, but does so 

 when there is an excess of citric acid. These constitute my group B. As 

 in the other group, any absorption which extends continuously from the 

 extreme blue end is not altered, but the detached absorption in the green 

 is almost immediately removed ; and therefore a deep pink or red solution 

 may at once become quite colourless, or only a very pale yellow ; and in 

 many cases this residual colour is due to some yellow colouring-matter 

 mixed with the other. I have never seen a colour which was changed by 

 sulphite when alkaline, and not when acid ; and thus citric acid never 

 restores the colour when it is added to the changed ammoniacal solution. 

 Excess of ammonia usually restores the faded acid solution to nearly the 

 original colour, and it is therefore not a case of actual decomposition, but 

 merely the result of some remarkable molecular change. A third group of 

 colours consists of those which are not almost immediately changed by 

 sulphite of soda, when either alkaline or acid ; and these I call group C. 

 Some of them may fade on keeping several hours, and some do not fade 

 even in several days, but they cannot thus be divided into two definite groups. 

 When thus faded, ammonia does not restore the colour ; and therefore it 

 is evidently the effect of decomposition, and not like the mere molecular 

 change met with in group B. 



On the whole, the groups A, B, and C are remarkably distinct. There 

 are, indeed, a few cases where the change takes place somewhat slowly ; 

 and a few scarlet colours do not show very distinctly the characteristic p 

 culiarities of either B or C ; but there are other very strong reasons for 

 believing that some of these are really mixtures of different groups. Even 

 if it should be found that perfectly simple colouring-matters may have, as 

 it were, intermediate characters, such appear to be so rare that practically 

 they may be classed with mixtures, until some reason be found for classing 

 them together as exceptions. 



These reactions of sulphite of soda are so much interfered with by the 

 presence of alcohol, that it should never be employed as a solvent, unless 

 the substance is insoluble in water ; and then it should be diluted as much 

 as possible, since the ordinary spirit of vvdne with an equal quantity of 

 water is the extreme strength admissible, and even that very much delays 

 the reaction. The effect of various other reagents is also sometimes very 

 different, according to the nature of the solvent. 



The three groups A, B, and C differ in other particulars. It is easy 

 to change A or B into C by various reagents which produce decomposi- 

 tion, but I do not know a case where C can be changed into A or B. Caustic 

 alkalies usually soon decompose colours belonging to group A, when dis- 

 solved in water, but act slowly on those of groups B and C. Usually 

 colours of group C are far more permanent than those of groups A and B, 



