224 HOW TO WORK 



the effect of different thicknesses. Coloured glasses cut wedge- 

 shaped are also very interesting, and are useful to compare with blow- 

 pipe beads ; but for actual research, no branch of the subject is so 

 satisfactory, as the testing of minute quantities of animal and 

 vegetable substances in the small cells. This includes the detection 

 of blood-stains, which can be doae with great ease and certainty 

 (Quart. Jouru. of Science, vol. II, p. 198) ; the detection of adultera- 

 tion in drugs and other substances met with in commerce ; and the 

 determination of the identity of, or the difference between, the very 

 numerous colouring matters met with in plants. In a numker of 

 such practical questions, special methods may be employed ivith 

 advantage ; bust in examining an unknown colouring matter, it is well 

 to adopt a' definite system, so as to be able to decide to what par- 

 ticular group it belongs. After a large number of experiments, I 

 found that it is .easy to arrange them in divisions founded on their 

 solubility in water or alcohol. Thus 



Division. 



Soluble in water and not precipitated by alcohol ... i 

 Soluble in water but precipitated by alcohol ... 2 



Insoluble in water but soluble in alcohol ..... ... 3 



Insoluble in water and alcohol ... ... ... 4 



Then we may divide i, 2, and 3, into groups, founded on the action 

 of sulphite of soda. The effect of this reagent is very remarkably 

 related to the spectra. If the absorption extends from the blue end 

 continuously, it produces no change, but if there is a detached 

 absorption in the green or yellow separated from the blue end by a 

 more transparent space, the sulphite in certain groups of colours 

 removes this, and leaves the absorption in the blue unchanged. In 

 some colouring matters this occurs in an ammoniaeal solution, and 

 these constitute my group A. In others, no such change takes 

 place unless the solution be strongly acid, and these form my group 

 B. This is usually quite independent of decomposition, and the 

 colour is restored by the addition of ammonia. Those colours which 

 are not immediately altered when the solution is acid, constitute 

 my group C. By these reactions, mixtures of colours of the 

 different groups can easily be recognised ; and this alone may often 

 be of great practical use. Then, in order to -divide these into sub- 

 groups, I have recourse to the number of distinct absorption bands, 

 when the neutral colour is dissolved in water or alcohol, or when 

 ammonia is added to each. By this means we obtain a large number 

 of sub-groups, which are of great use in practical researches. It 

 would extend this account to an unreasonable length, if J were to 



