CLASSIFICATION OF PROTEIDS. 49 



molecule, it can be easily displaced by a nickel salt, and then in turn by a 

 copper salt, each yielding its characteristic colour reaction. He examined 

 these and other reactions in connection with various albuminoids as well ; the 

 addition of cobalt sulphate and potash to gelatin he found to produce a play 

 of colours in the order of those of the spectrum, commencing with violet. 



Drechsel l has drawn attention to an old observation of Krukenberg's, that 

 at the boiling temperature there is in the so-called biuret reaction a reduction 

 of the cupric to cuprous oxide ; the latter, however, remains in solution. 

 Drechsel shows that the reduction also occurs at the ordinary temperature. 

 C. J. Martin 2 is also of opinion that the biuret reaction is a reduction. 

 He rinds that alkali albumin dissolves cuprous oxide and forms a pink solution, 

 never violet or purple ; these latter colours, when the test is ordinarily performed 

 with copper sulphate, are due to admixture with cupric hydrate, held in solu- 

 tion by the proteid and not reduced. The most powerful reducing proteids are 

 proteoses and peptone, hence the pink biuret reaction ; whereas the native 

 proteids have a smaller reducing power, and the pink colour is mixed with the 

 blue cupric hydrate, and so the colour obtained is a violet. 



From the preceding study of the properties and reactions of the 

 proteids, it will be gathered that since many other substances give the 

 same tests, a proteid can only be identified by employing a large 

 number of its reactions. Winternitz 3 recommends a combination of a 

 precipitant and colour reactions. The precipitant he has chiefly used 

 in cases of albuminuria is acetic acid and potassium ferrocyanicle. The 

 precipitate so obtained gives the colour reactions well. This is also the 

 case with the precipitate produced by several other reagents, among 

 which may be mentioned salicylsulphonic acid, 4 and the halogens. 5 



CLASSIFICATION OF PROTEIDS. 



It will be seen from the preceding description of the proteids, that 

 I have used the term proteids throughout as an equivalent for albuminous 

 substances (German, Eiweisskorper) ; certain other substances (such as 

 haemoglobin, mucin, nucleo-proteids) named proteids, by Hammarsten, 

 Neumeister, and other continental writers, will be treated separately 

 as compound proteids. 



The proteids may be divided into those of animal and those of 

 vegetable origin. There does not appear to be any essential difference 

 between these two classes, and each can be subdivided in the same 

 manner into sub-groups, but the distinction is a convenient one in practice. 



Animal proteids. Class 1. Albumins. These are proteids which 

 are soluble in water, in dilute saline solutions, and in saturated solutions 

 of sodium chloride and magnesium sulphate. They are, however, pre- 

 cipitated by saturating their solutions with ammonium sulphate. Their 

 solutions are coagulated by heat, usually at 70-73 C. Serum albumin, 

 egg albumin, lact-albumin are examples. 



Class 2. Globulins. These are proteids which are insoluble in water, 

 soluble in dilute saline solutions, and insoluble in saturated solutions of 

 sodium chloride, magnesium sulphate, and in half -saturated solution of 



1 Ztschr. f. physiol. Ohem., Strassburg, 1895, Bd. xxi. S. 68. 



2 Private communication to author. 



3 Ztschr. f. physiol. Chem., Strassburg, Bd. xv. S. 187; xvi. S. 439. 



4 Pickering, loc. cit., p. 377. 



5 F. G. Hopkins, Proc. Physiol. Soc., June 12, 1897. 

 VOL. I. 4 



