ELEMENTS OF COMPOSITION. 13 



further modifications) to be looked upon as effete, and no longer service- 

 able for any of the functions of life. As such, they may either circulate 

 in the various juices of the body until eventually excreted, or may 

 remain behind in the tissues in the character of dregs or sediment as it 

 were. 



All protein substances are of exceedingly complex composition. They 

 contain, beside carbon, hydrogen, and oxygen, a large amount of nitro- 

 gen, and invariably sulphur. Phosphorus was also formerly supposed, 

 though erroneously, to be present. Their true constitution is still quite 

 obscure. 



They all become swollen and puffy when placed in water, and enter 

 into combinations with acids and bases, but whether in regular propor- 

 tion is not yet known. They dissolve in alkalies, but probably with meta- 

 morphosis or decomposition, and may be thrown down from such solu- 

 tions by the mineral acdds. They likewise form combinations with acids, 

 from which they may be again precipitated by means of the alkalies. 

 The action of nitric acid causes them to assume a yellow hue, from the 

 generation of an acid known as xantlioproteinic. Millon's reagent also, 

 a solution of nitrate of mercury, containing nitrous acid, communicates a 

 red colour to them, while iodine tinges them yellowish-brown. In con- 

 centrated hydrochloric acid they are dissolved, assuming at the same 

 time a violet tint. The action of sugar and concentrated sulphuric acid upon 

 the protein substances gives rise to a change of colour in them, at first to 

 purple, and subsequently to more of a violet-hue (Schultze) a reaction 

 which they share with the acids of the bile and with elain. In watery 

 solutions they bend a ray of polarised light to the left. Oxidizing agents, 

 as well as dry distillation and putrefaction, develope in albuminous 

 bodies a number of decomposition-products, such as formic, acetic, and 

 benzoic acids, oil of bitter almonds, and also crystalline matters, as 

 leucin and tyrosin. (See below.) 



Most of the protein substances appear in the body under two isomeric 

 modifications firstly, in solution, or gelatinised, as in the greater number 

 of fluids and tissues of the system ; and secondly, in a coagulated or in- 

 soluble state. They pass from the former into the latter condition in 

 various ways, partly by boiling, partly by the action of strong acids, and 

 finally, as the saying is, spontaneously. In the first modification the 

 protein substances may be far more easily distinguished, one from the 

 other by certain definite reactions, than when in the coagulated con- 

 dition. 



9- 



The complex composition of the principles under consideration, their 

 indifferent nature, and great instability, account for the fact that, up to 

 the present, their true constitution has remained utterly unknown. 

 Indeed, we find a most discouraging obscurity resting over this most 

 important of all groups of animal substances. So far is this the case 

 that, in fact, we are not even able to enumerate the various albuminous 

 principles with anything like certainty. 



Again, the great instability of the protein substances gives rise to the 

 appearance in the organism of a considerable number of decomposition- 

 products, to whose nature and mode of origin we are still in most cases 

 almost complete strangers. Among these may be reckoned, as far as we 

 know at present, urea, uric, hippuric, and gallic acids; taurin, glycin, 



