212 THE DETECTION OF ADDED SUBSTANCES. 



of formaldehyde, and keeping these and the tubes of the milks 

 to be tested at a temperature of 37 C. ; it is also advisable to 

 dilute the milk 10 and 100 times, prepare tubes from the diluted 

 milk, and keep these at 22 C. The controls kept at 37 may 

 contain 0-005, 0-003, and 0-001 per cent, formaldehyde, and 

 those kept at 22, 0-001, 0-0008, 0-0005, and 0-0003 per cent. 



By noting which of the control tubes is decolourised at the 

 same time as the sample to be tested, a fairly accurate estimation 

 of the amount of formaldehyde present may be made. 



The colour of the fat is of some aid in judging the amount of 

 cream abstracted ; if it is very yellow, the milk very likely is 

 yielded by Jersey cows, and a high figure e.g., 4 may be 

 expected. 



The colour of the milk itself is no guide, as it frequently is 

 artifically coloured to give it an appearance of richness. Annatto 

 was the colouring-matter chiefly used, but this is now somewhat 

 largely replaced by coal-tar colours, especially the sodium salt 

 of di-methyl-amino-azo-benzene sulphonic acid or methyl orange. 

 Artificial colouring-matters generally may be detected by pre- 

 cipitating the casein with acetic acid, washing well with water, 

 and digesting with strong alcohol ; the casein carries down the 

 colouring-matter and gives it up to the alcohol ; on evaporating 

 this, and taking up with a little water, the colour can be detected. 

 Annatto is unchanged by mineral acids, while many of the coal- 

 tar colours turn pink. 



The following method for the detection of colouring-matters 

 in milk is based on a scheme devised by M. Wynter Blyth : 



Preliminary Tests. (1) Allow a portion of the milk to stand 

 in a cool place till the cream rises ; if the skim milk is more 

 highly coloured than the cream the milk is artificially coloured. 



(2) Add a drop or two of hydrochloric acid to a little milk ; 

 a pink colour indicates the presence of an azo colour, of which 

 the following, among others, may occur in milk : 



Aniline yellow. Amino-azo-benzene. 



Butter-yellow. Chrysoidine. Di-methyl-amino-azo-benzene. 

 Acid yellow. Salts of amino-azo-benzene sulphonic acid. 

 Methyl-orange. Salts of di-methyl-amino-azo-benzene sulphonic 



acid. 

 Orange IV. Diphenylamine-yellow. Salts of di-phenylamine-azo- 



benzene sulphonic acid. 



(3) Make the milk alkaline with sodium bicarbonate, and 

 immerse a strip of filter paper therein for at least twelve hours. 

 A reddish-yellow stain indicates annatto. 



These tests may fail to show artificial colouring-matters, 

 because (1) aniline-yellow and butter-yellow are soluble in fat, 

 and may rise, with the cream ; (2) azo-compounds are reduced 

 n stale milk to colourless compounds ; and (3) a colour such as 



