THE NITROGENOUS SUBSTANCES 73 



(tyrosin). (3) The reaction of Adamkiewicz gives a reddish violet color with 

 1 vol. concentrated H 2 SO 4 and 9 vol. acetic acid containing some glyoxylic acid, 

 and characterizes the indol group (tryptophan). (4) By means of Molisch's 

 reaction, which is a violet color with concentrated H 2 SO 4 and a-naphthol, the 

 carbohydrate groups are demonstrated, and (5) by boiling with alkali and a 

 lead salt (formation of black lead sulphide) the cystin groups are detected. 



The biuret reaction gives a color changing from red through reddish violet 

 to violet blue, on addition of dilute solution of copper sulphate to a proteid 

 solution previously alkalized with caustic potash or soda and then warmed. It 

 is very generally regarded as specifically characteristic for the proteids. This 

 reaction, according to H. Schiff, appears with those substances which contain two 

 CO.NH 2 , CS.NH 2 -, C(NH).NH 2 -, or, under certain conditions, -CH 2 .NH 2 - 

 groups, joined together by their own C atoms, or by a C atom in a CH-, 

 CH(OH)-, CO group, or finally by a N atom in an NH group. However, this 

 reaction is not a positive criterion of the proteid nature of a body, for on the 

 one hand substances of so simple a structure as the glycinamid give it, and on 

 the other, it is wanting with the desamido-albumin (obtained by the action of 

 nitrous acid) where all the carbon nuclei of the proteid are present. 



As a matter of fact there is at this time no physical or chemical feature 

 by which one can decide positively whether a compound is to be described as 

 a proteid or not ; and for this reason certain substances are by some enumerated 

 among the proteids, while by others their proteid nature is positively denied. 



After discussing exhaustively all the facts obtained from the cleavage 

 products of proteids,, Hofmeister reaches the conclusion that proteids arise 

 chiefly by condensation of the a-arnino acids, union taking place regularly 

 and repeatedly by means of the CO-NH-CH = groups. He remarks, how- 

 ever, that this conception of the subject does not explain all forms of linkage 

 in proteid and that, considering our incomplete knowledge of the proteid 

 molecule, other relations are by no means excluded. 



A rational classification of the simple proteids can only be carried out 

 when we possess more exact knowledge of their constitution, and are thus 

 able to state what nuclei occur in each individual proteid and in what quan- 

 tity. This is far from possible as yet, and with most proteids we are not 

 even in position to say whether they are actually chemical individuals or are 

 mixtures of different substances. We are compelled therefore to classify the 

 proteids according to their relative solubility and precipitative reactions. This 

 is by no means a scientific principle of classification, but it is justifiable on 

 purely practical grounds. 



Accordingly simple proteids have been divided into the following groups: 



A. Native proteids. These are obtained from the tissues and fluids of the 

 body by neutral chemical reagents: the albumins, globulins and mucins are 

 the most important. 



1. Albumins: soluble in water; not precipitated from aqueous solutions by 

 small quantities of acids or alkalies, but precipitated by larger quantities of 

 certain acids and metallic salts. On boiling, the solutions are coagulated if 

 salts are present. They are precipitated by NaCl or by MgSO 4 only on addition 

 of acetic acid. They are not salted out by half-saturation of their solutions 

 with ammonium sulphate, but are so separated with greater concentration of 

 the salt. 



