( V.. ISSZJVi 'A TION OF J'ROTEIDS. 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. lie examined 
these and other reactions in connection with various albuminoids as well ; the 
addition of cobalt sulphate and pol ish to gelatin he found to produce a play 
of colours in the order of those of the spectrum, commencing with violet. 
Drechsel 1 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, 
lb' finds that alkali albumin dissolves cuprous oxide and forms a pink solution, 
never violet or purple ; these latter colours, when the test i< 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. AVinternitz 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 ferroeyanide. 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, 
Xeumeister, 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. Album ins. — 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 c -73° C. Serum albumin, 
egg albumin, lact-albumin are exampi<-. 
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 Ztsehr. f. physiol. Chem., Strassburg, 1895, Bd. xxi. S. 6S. 
2 Private communication to author. 
3 Ztschr. f. physioI. Chem., Strassburg, Bd. xv. S. 187; xvi. S. 489. 
4 Pickering, Joe. cit., p. 377. 
5 F. G. Hopkins, Proc. Physiol. Soe., June 12, 1897. 
VOL. I. — 4 
