CHAP. I.] THE PROTEIDS. 13 



SEC. 2. CHEMICAL REACTIONS CHARACTERISTIC OF THE 

 PROTEIDS \ 



Only certain of the proteids are soluble in water; they are all 

 soluble however, especially with the aid of heat, in concentrated acetic 

 acid, and in solutions of the caustic alkalies; they are insoluble in 

 cold absolute alcohol and in ether. 



Solutions of the proteids are precipitated by the following 

 reagents: 



1. By strong mineral acids added in sufficient quantities. 



2. By acetic acid and potassium ferrocyanide. 



3. By acetic acid and a large addition of concentrated solutions 

 of neutral salts of the alkalies or alkaline earths. 



4. By basic lead acetate. * <,,, JU.*^. / '-, 



5. By mercuric chloride. ~t n.J .., ..^ q^^J&ft . 



6. By tannic acid. 



7. By powdered potassium carbonate added to the solution until 

 it is nearly saturated. 



8. The majority of the proteids are completely precipitated from 

 their solutions by alcohol, though in the presence of free alkali they 

 are slightly soluble in hot alcohol. 



Detection When proteids are present in a solution the following 



of Proteids reactions are employed in their detection : 

 in solution. j ^ Uquid ig boiled and nitric ^ ^^ g() ag 



to produce a strong acid reaction. The occurrence of a precipitate on 

 boiling, which is undissolved by nitric acid, and the immediate pro- 

 duction of a precipitate by nitric acid indicates the presence of a 

 proteid, to be confirmed by other tests. 



2. The liquid is rendered strongly acid with acetic acid, and 

 solution of potassium ferrocyanide added; all proteids are thrown 

 down in the form of a white flocculent precipitate. 



3. The liquid is rendered strongly acid with acetic acid, and is 

 boiled with its own volume of a saturated solution of sodium sulphate, 

 which will precipitate any proteid present. 



The above tests are very satisfactory except in the case of only 

 slight traces of proteids being present ; under any circumstances it is 

 desirable to obtain confirmatory evidence ; the following methods are 

 then useful : 



4. Millon's reaction. When a strongly acid solution of mercuric 

 nitrate, made according to the directions to be afterwards given, is 



1 In preparing a part of this section the author has availed himself greatly and 

 followed very closely, in some sentences almost literally, 135 and 136 of Professor 

 Hoppe-Seyler's Handbuch der physiologisch- und pathologisch-chemischen Analyse. 

 3 rd Edit. 1870. 



