196 MARKET DAIRYING 
the same; if boric acid is present, the glycerin tube will 
be lighter, usually white. 
2. Another method commonly used is to treat about 
too c.c. of milk with enough sodium or potassium 
hydroxide to render the same alkaline; then evaporate 
to dryness and incinerate; dissolve the ash in water 
slightly acidulated with hydrochloric acid, and filter; 
dip a piece of turmeric paper in the filtrate and dry at 
212° F. If boric acid is present, the paper will assume 
a reddish brown color. 
Formaldehyde. ‘This is a violent poison which has 
strong antiseptic properties. A trace of it will pre- 
serve milk for a considerable time. As in the case 
of boric acid, a number of methods are available for 
detecting small quantities of this preservative in milk. 
The following method will reveal the preservative, even 
when only a trace of it is present. 
Mix 5 c.c. each of milk and water in a porcelain dish. 
Add to c.c. of hydrochloric acid containing a trace of 
Fe, cl,, then heat the mixture very slowly. A _ violet 
color is formed in the presence of formaldehyde. 
Another method of detecting formaldehyde is as fcl- 
lows: Mix equal quantities of milk and water in a 
test tube, then add a little of 90 per cent commercial 
sulphuric acid in such a way that the acid will run along 
the side of the tube and form a layer at the bottom. 
In the presence of formaldehyde, a bluish or violet zone 
develops at the junction of the acid and the milk; in 
its absence the zone develops a greenish color. This 
method will detect one part of formaldehyde in 200,000 
parts of milk. 
Salicylic Acid. This acid, while less poisonous than 
formaldehyde, seriously affects digestion and also exerts 
