90 



Mr. E. Scliunck on the Colouring and 



[EecesSj 



acids, indigo-red and indigo-blue, can be extracted," and that "it yields 

 neither indigo-red nor indigo-blue by boiling with acids/' This result, 

 however, may easily be accounted for by any one conversant with the 

 subject who attentively considers the details of his process. The indigo- 

 producing body of urine, if it be not identical withindican, is certainly quite 

 as susceptible of change as the latter ; and the small quantity existing in 

 the secretion may easily disappear under the influence of heat, alkalies, or 

 ermentation, and become so changed as no longer to yield indigo-blue or 

 indigo-red with acids. The nature of this change I have explained in my 

 papers on the formation of indigo-blue. Now, Dr. Thudichum's process 

 comxmenees by adding to urine an excess of caustic baryta or lime. At 

 subsequent stages he boils and evaporates his liquids with the assistance of 

 heat. After operations such as he describes it is impossible that any trace 

 of indican, or any body resembling it, can remain undecomposed. Unless 

 certain precautions are adopted in conducting delicate experiments, only 

 negative results can be expected. 



The alJcaptone of Baedeker'^, and the colloid acid from urine lately 

 described by Marcet f, probably stand in some relation to the ordinary ex- 

 tractive matters of urine, which they strongly resemble in most of their 

 chemical and physical properties. The nature of the methods employed 

 for the preparation of these bodies renders it, however, extremely doubtful 

 whether they preexisted in the secretion, since in both cases solutions 

 containing, together with the organic substances, strong mineral acids, were 

 heated and even evaporated — a proceeding which must have led to the de- 

 composition of the extractive matters, and the formation of bodies not pre- 

 viously existing. 



The preceding account, in which I have endeavoured to present a sum- 

 mary of the results obtained in previous researches, will serve to give an 

 idea of the present state of our knowledge on this subject ; and I will now 

 proceed to give an account of my own experiments. Before doing so, I may 

 state that I shall apply the term "colouring-matter" to those bodies only 

 which occur naturally in urine, or are formed by processes of decomposition, 

 and which are insoluble, or not easily soluble, in water. The substances 

 easily soluble in water, to which the colour of normal urine is due, I shall 

 continue to call " extractive matters," until I shall have shown that they 

 are bodies the properties and composition of which are sufficiently definite 

 and unvarying to justify me in bestowing on them peculiar names. 



The extractive matters being, as I believe, the source from which most 

 of the colouring-matters of urine are derived, I resolved to commence the 

 investigation by a careful examination of their properties and composition. 

 The first step, indeed, which I thought it necessary to take before proceed- 

 ing further at all was to ascertain whether these extractive matters are 



* Annalen der Chemie vmd Pbarmacie, B, cxvii. S. 98. 

 t Proceedings of the Eoyal Society, vol. siv. p. L 



