ELEMENTS IN ORGANIC COMPOUNDS 189 
point will precipitate the halids of silver which have the characteristic 
appearance. 
Tests for Bromine and Iodine. Acidify a few cubic centimeters of 
the sample and boil to remove He2S and the cyanids. Add a few drops 
of carbon bisulphide and a little chlorine water. If iodine is present, 
it is taken up by the heavier bisulphide which settle to the bottom 
of the tube with a distinct violet color. 
Nal+H3804= HI-+NaHS0Ou. 
2H1I-+Cls = H40+Io. 
Qualitative Tests for Elements in Proteins. Carbon. Heat the 
protein on a piece of platinum foil and if charring takes place the pres- 
ence of carbon is indicated. 
Hydrogen. Test for water in the top of the test tube. 
Nitrogen. Heat with soda lime and test for ammonia by smell and 
moist litmus paper. 
Sulphur. Heat with concentrated potassium hydroxide which will 
set free potassium sulphide. Cool and add lead acetate solution, which 
will precipitate black lead sulphide. After adding the lead acetate solu- 
tion acidify, boil and suspend lead acetate paper above the liquid. 
Detection of Proteins. (1) The coagulation of proteins by heat 
especially in slightly acid solutions may be used in their detection. 
This precipitate does not disappear on continued addition of acid. 
(2) Alcohol in excess will form a precipitate which is soluble in water 
at first; if the precipitate is left in contact with the alcohol, it is changed. 
The proteins come down unchanged but are probably dehydrated when 
left in contact with the alcohol. The fixing of tissue in histological work 
rests upon this fact. (8) Heller’s Ring Test rests wpon the fact that 
strong mineral acids will coagulate protems. A small amount of con- 
centrated nitric acid is put into a test tube and some of the test solution 
is floated upon it. If proteins are present a whitish layer will be formed 
at the point of contact between the two liquids. 
Action of Bacteria on Proteins. The hydrolysis of proteins by 
bacteria has been studied by different investigators with the production 
of data which is not in agreement. Bainbridge (1911) found that bac- 
teria could not decompose “ purified native proteins.” This was later 
verified by Sperry and Rettger (1915) who worked with purified serum 
albumin, egg albumin and edestin and the common proteolytic bacteria. 
They reported that when all other forms of nitrogen were removed, 
