and Vegetable Colouring -matters by the Spectrum Microscope. 165 



is to distinguish the separate colours, if we are not sure that they are 

 pure and simple. Sometimes special facts may be of use ; but, as a 

 general rule, we are compelled to have recourse to the position and 

 character of the general absorption. This requires a good deal of 

 care, since a difference in the state of the solution may make the 

 same colour differ more than two quite distinct colours. After trying 

 a number of experiments, I find that the best spectra for comparison 

 are those obtained by adding first a moderate excess of carbonate of 

 soda, and then a considerable excess of citric acid. Both of these 

 solutions change very slowly, and give well-marked spectra ; whereas 

 ammonia often causes decomposition, and weaker alkalies or acids 

 give much more faint spectra, or such as rapidly fade. Closely re- 

 lated colours should be compared together, and made as nearly equal 

 as possible after the addition of the carbonate, and then citric acid 

 added in considerable, and nearly equal, excess. We thus can com- 

 pare two different spectra ; and even if the position of the absorp- 

 tion is the same in both cases, the relative intensity may vary consi- 

 derably. Very closely allied colours may often be easily distin- 

 guished in this manner ; and the only great difficulty is when coloured 

 impurities are present. As an example, I give some colours belong- 

 ing to subgroup 1, B, aq am al am . 



Carbonate of soda. Citric acid. 



Petals of Wall flo wer ( Chei- \ o1 K q n in 03 7 



ranthus Cheiri) |^.--.5 S..9--1U— ^..--..7 



Dark grapes 2| 5|...9 10.. 11-- 4£..--..8| 



Fruit of Elder (Sam-\ x - n 41 .. 8 r 



bucus nigra) J 2 ' * 2 ' ' 2 



The first differs entirely from the latter two ; but they are so similar 

 that it requires great care to be sure that they differ essentially. If 

 it were quite certain that such colours were pure, it would not be 

 difficult to distinguish them with confidence ; but since they may 

 contain coloured impurities, we must occasionally be content with 

 results somewhat doubtful in questions of minute detail, which might 

 not be of the least importance in some practical investigations. 



24. Yellow Colours. 



One of the best general methods of distinguishing yellow colours 

 belonging to subgroup C, aq am al am , or those with bands which 

 are much alike, is to make them as nearly as possible of the same 

 tint when neutral, and then to add excess of ammonia, which may 

 make them very unequal. For example — 



Neutral. Ammonia. 



Yellow Dahlia (D. variabilis) 8 . . 9 - - 10— 3. . .4 - - 4|— 



Yellow Calceolaria ( C. aurea-floribunda) 7 ... 9 - - 1 1 — 6 J . . 6| - - 7 — 



Saffron {Crocus sativus) 7...8--11 — 7.. 8 --11 — 



The action of ammonia thus shows that they differ very much, but at 

 the same time the Calceolaria might be a mixture of the other two ; 

 and this would have to be decided by other facts. 



