and Vegetable Colouring-matters by the Spectrum Microscope. 151 



at command. My aim has been to contrive a special system of 

 qualitative analysis of coloured substances applicable to minute quan- 

 tities, and as independent of general chemistry as the blowpipe method 

 is in the case of minerals. I may here say that, in some very important 

 practical applications to the detection of blood-stains, not above 

 ■j-^0- of a grain was at disposal, and yet perfectly satisfactory results 

 were obtained. 



I was led to study the colouring-matters of flowers, leaves, fruits, 

 woods, and roots, because it appeared a most admirable field of 

 inquiry to teach the general principles of the subject. The colours 

 being so various, and occurring under such complicated conditions, 

 I thought that if methods could be devised to distinguish those 

 that are dissimilar and to prove the identity of those that are alike, 

 even when mixed with coloured impurities, such principles could easily 

 be applied to other inquiries. If the question were merely to distin- 

 guish or compare absolutely pure colouring-matters, there would be 

 little or no difficulty ; but it appeared to me that one great value of 

 the method would be to be able to apply it at once to very impure 

 and mixed materials. In such cases mere colour is of very secondary 

 importance, since that may be totally changed by a very small amount 

 of impurity. 



7. Preparation of Colours. 



If the petals, leaves, &c. of plants be crushed in water, it very 

 commonly happens that the colour is rapidly decomposed and 

 no clear solution can be obtained; but if crushed in a mode- 

 rate quantity of spirits of wine, and the solutions queezed out, fil- 

 tered, and evaporated to dryness at a gentle heat, the colouring- 

 matter does not decompose, even when redissolved in water and 

 filtered to remove anything not soluble in that liquid. This clear 

 solution should then be evaporated to dryness at a gentle heat in a 

 small saucer, and kept dry ; for then the colours often undergo no 

 important change in the course of many months, whereas, when kept 

 dissolved in water or alcohol, they may quickly decompose. I have 

 thus prepared the colouring-matter of above a hundred different 

 vegetable substances, some of which have become entirely changed, 

 but a large number are apparently still unaltered. I have also kept 

 a number of colours, sealed up in glass tubes, ready for direct exami- 

 nation, dissolved in alcohol, in strong syrup, or in alum. Many 

 have decomposed ; but many have kept perfectly well, or have merely 

 faded, and still give excellent spectra after above a year. I have 

 also prepared and kept in the same manner some animal colouring- 

 matters, but comparatively few. 



8. Method of Examination. 

 The coloured substances are examined, when dissolved in water, 

 alcohol, or other solvent, in the small glass cells already described ; 

 and the various reagents are added and mixed by means of a mode- 

 rately stout platinum wire, flattened at one end and turned up square, 

 like a little hoe. I have made many experiments in order to ascer- 

 tain what reagents are most serviceable in developing characteristic 



