THE DAIRY 199 



Formaldehyde may be detected in milk by adding to a small 

 quantity of the milk an equal volume of strong hydrochloric 

 acid containing about 2 per cent, of a 10 per cent, solution of 

 ferric chloride. The mixture is heated gradually to the boiling- 

 point, when a violet colouration is produced if formaldehyde be 

 present. Pure milk slowly becomes brown under this treat- 

 ment. 1 part of formaldehyde in 250,000, it is said, can be 

 detected by this test. 



Boric acid and borax have long been used for preserving 

 milk in hot weather. They are not nearly so effective as 

 formaldehyde, and have to be added in much larger propor- 

 tions; thus 1 part of a mixture of boric acid and borax 

 added to 2000 parts of milk had scarcely any preservative 

 action at 20°, 1 in 1500 extended the period before curdKng 

 from 36 to 66 hours, 1 in 1000 to 72 hours, and 1 in 500 to 

 96 hours. 



Boric acid can be detected by ashing the milk (best after 

 addition of lime), acidifying with a little dilute hydrochloric 

 acid, and immersing in the liquid a strip of turmeric paper. On 

 drying, the paper turns red if boric acid be present, and on 

 moistening with very little caustic soda, takes a greenish-black 

 colour. 



The other preservatives are rarely used. 



Products Derived from Milk. 



The following substances must be very 'briefly considered 

 here : 



Cre^m. — The fat, being specifically lighter than the aqueous 

 portion of milk (sp. gr. of fat at 15° =. 0-930, that of the rest 

 of the milk about I'OSG), tends to rise to the surface. The 

 resistance to the motion of the small globules is large, while 



