CHEMICAL BASIS OF THE ANIMAL BODY 7 



can however be attached to these formulae, for, as Drechsel observes, in so large a 

 molecule an analytical error of '01 p.e. would have the same importance as would 

 one of '1 p.c. in ordinary analyses. They give us at most an idea of the minimal 

 magnitude of the proteid molecule, but apart from this they throw no more light on 

 the subject than already existed in Lieberkiihn's older formula. 



General reactions of the protends 1 . 



1. Heated with strong nitric acid, they or their solutions turn 

 yellow, and this colour is, on the addition of ammonia, or caustic soda 

 or potash, changed to a deep orange hue. (Xanthoproteic reaction.) 



If much proteid, except albumoses and peptones, be present a yellow 

 precipitate is obtained at the same time. With less proteid their 

 solutions merely turn yellow on boiling and orange on the addition of 

 the alkali : if only a trace of proteid is present no yellow colour is 

 observed until after the addition of the alkali. 



2. With Millon's reagent 2 they give, when present in sufficient 

 quantity, a precipitate, which turns red on heating. If they are only 

 present in traces, no precipitate is obtained, but merely a red colouration 

 of the solution when heated. 



3. If mixed with an excess of concentrated solution of sodium 

 hydrate, and one or two drops of a dilute solution of cupric sulphate, 

 a violet colour is obtained, which deepens in tint on boiling. (Piotrowski's 

 reaction 3 .) 



The above serve to detect the smallest traces of all proteids. 



4. Render the fluid strongly acid with acetic acid, and add a few 

 drops of a solution of ferrocyanide of potassium ; a precipitate shews 

 the presence of proteids, except true peptones and some forms of 

 albumose. 



5. Render the fluid, as before, strongly acid with acetic acid, add 

 an equal volume of a concentrated solution of sodium sulphate, and 

 boil. A precipitate is formed if proteids, except peptones, are present. 



This reaction is particularly useful, not merely because it effects a very complete 

 precipitation of the proteids which are present (except peptones) but also because 

 the reagents employed do not produce any decomposition of other substances which 

 may be present, and do not interfere with certain other tests which it may be 

 necessary to apply after the removal of the proteids by filtration. It is of use more 

 particularly in the determination of sugar in blood 4 . 



1 Consult in all cases Hoppe-Seyler's Hdbch. d. physiol. path. chem. Analyse. 

 Ed. v. 1883. See also Krukenberg, Sitzb. d. Jena. Gesell. f. Med. u. Natwiss. 

 1885, Nr. 2. 



2 Compt. Rend. T. xxvm. (1849), p. 40. 



3 Sitzb. d. Wien Akad. Bd. xxiv. (1857), S. 335. 



4 See Gamgee's Physiol. Chem. Vol. i. p. 195. 



