DR. EDWARD SCHUNCK ON INUIGO-BLUE. 219 



yielding plants contain ready-formed indigo-blue (as has 

 been maintained with so much persistence), or whether 

 the colouring-matter exists in the vegetable cells in the 

 form of indican or some other glucoside ; and I have ac- 

 cordingly examined such of the plants known to give indigo 

 as I have been able to procure. 



Before stating the results to which I wish to direct at- 

 tention on this occasion, I may mention an observation 

 belonging, strictly speaking, to the part of the subject 

 previously treated of, which, however, I will now describe 

 in a few words, as I may not have another opportunity of 

 doing so. In my last memoir I stated that, among the 

 products of decomposition of indican from Isatis tincta- 

 ria, leucine is usually found, sometimes, indeed, in con- 

 siderable quantities. In some more recent experiments 

 made with woad leaves I obtained, besides leucine, a sub- 

 stance having all the properties of tyrosine. This substance 

 was only slightly soluble in cold water, but soluble in 

 boiling water, from which it separated on cooling in long 

 needles, forming, when dry, a snow-white felted mass. 

 Its watery solution gave the well-known reaction with 

 mercuric nitrate. Its solution in concentrated sulphuric 

 acid, after neutralization with barium carbonate, gave a 

 purple colour with ferric chloride. According to Prof. 

 Gamgee, who had the kindness to examine the substance 

 for me, it showed under the microscope the forms charac- 

 teristic of ordinary tyrosine. There could be no doubt, 

 therefore, of its identity with the latter. As the tyrosine 

 in this case was not obtained from pure indican, but from 

 the crude alcoholic extract of the leaves, it is impossible to 

 say whether it preexisted in the plant or whether it was 

 a product of decomposition of the indican of the extract. 

 The latter supposition is the most probable one; for tyrosine, 

 being almost insoluble in alcohol, could hardly be contained 



