328 PROTEIN-COMPOUNDS. 



protein-compound. The greater number cannot be precipitated 

 from their aqueous solution by alkalies or by most of the vegetable 

 acids, but they are precipitated by mineral acids (with the exception 

 of ordinary phosphoric acid) and by the tannic acids. 



Most of them are transformed into their insoluble state by 

 boiling, some by acetic acid, and almost all by the mineral acids ; 

 with the latter they usually form compounds soluble in pure water 

 but insoluble in water to which an acid has been added, and inca- 

 pable of being restored to the soluble modification by saturating 

 the acid with a base. The protein-compounds, when precipitated 

 by salts, usually assume the insoluble form. 



The insoluble compounds, when dried, are white and pulver- 

 isable ; when newly precipitated they are usually of a snow-white 

 colour, flocculent or in small clots, or else tough and gelatinous, 

 without taste or smell, without reaction on vegetable colours, and 

 insoluble in water, alcohol, ether, and all indifferent menstrua; 

 they are all more or less readily dissolved by alkalies, from which 

 they can be precipitated by mere neutralisation with acids. They 

 behave very differently towards different acids ; they are dissolved 

 by concentrated acetic acid and other organic acids, as well as by 

 ordinary phosphoric acid, and are precipitated from these solutions 

 by yellow as well as red prussiate of potash. They do not 

 dissolve in moderately concentrated mineral acids, although they 

 combine with them, and these compounds have the property of 

 being insoluble in water to which an acid has been added, although 

 they dissolve in pure water, after having first swelled and assumed 

 a gelatinous appearance. They swell in the same manner in con- 

 centrated sulphuric acid, but they assume at the same time a 

 brownish colour, and become decomposed. Their relation to con- 

 centrated nitric and hydrochloric acid is highly characteristic ; the 

 former acid giving them when heated a deep lemon-coloured tint, 

 while concentrated hydrochloric acid causes them to assume a 

 gradually increasing intensely blue colour, when exposed to a mode- 

 rate warmth and to a sufficient supply of air. A fluid obtained by 

 the solution of 1 part of mercury in 2 parts of nitric acid containing 

 4f equivalents of water, forms the most delicate test for the protein- 

 compounds, (Millon,*) whether they are dissolved in a fluid or 

 simply interspersed in a tissue. The fluid, or the tissue that has 

 been moistened with it, is then heated to from 60 to 100, when an 

 intense red colour is observed, which does not disappear either on 

 prolonged boiling or exposure to the atmosphere. 

 * Compt.rend. T. 27, p. 42-44. 



