208 THE DETECTION OF ADDED SUBSTANCES. 



To a few c.c. of milk add a little of this solution ; if /3-naphthol 

 is present a red colour will be produced. Do not make alkaline, 

 as milk itself gives a brownish-red in alkaline solution. 



As a confirmatory test, a diazotised solution of phenylhydrazine 

 may be used, which gives a red colour in alkaline solution with 

 /3-naphthol, but no colour with milk. 



If the milk is extracted with chloroform, and the chloroform 

 heated with caustic potash for a few minutes, a deep blue colour 

 indicates the presence of /?-naphthol. 



Fluorides are thus detected in the ash of milk. At least 

 25 c.c. of milk should be taken, and the ash treated in a platinum 

 basin with a little strong sulphuric acid. Over the top of the 

 basin a watch-glass coated with bees' wax, through which a 

 few lines are scratched, is placed, and a piece of ice or some cold 

 water is put into the concave depression. The basin is then 

 warmed gently and the watch-glass exposed to the action of the 

 fumes evolved for ten minutes'. In the presence of fluorides 

 it is seen that the glass has been etched, after removal of the 

 wax. If a drop of water is placed on the wax, away from the 

 lines scratched through it, a white film of silica will be formed 

 on its surface if fluosilicates be present. If fluoborates 

 be present, this drop of water will give a boric acid reaction ; in 

 the presence of fluoborates both a fluoride and a boric acid 

 reaction are given by the ash of the milk. 



0. and C. Hehner have pointed out that when there is much 

 boric acid in relation to the fluoride present, the test for fluorides 

 applied directly to the ash fails. The milk should be made 

 alkaline, ashed, and the ash dissolved in a little acid ; calcium 

 chloride is added, and the solution made alkaline with ammonia ; 

 the precipitate is collected, burnt, and extracted with acetic 

 acid, and the test made on the insoluble portion. 



Monier-Williams' test is given under Butter (p. 223). 



Formaldehyde, which has been introduced of late years, is 

 now frequently employed as a milk preservative. 



It is generally added as a 1 per cent, solution in water, which 

 is made by diluting the 40 per cent, solution known as " For- 

 malin," " Formal," " Formol," or " Formine." A very large 

 number of reactions for this substance have been worked out. 

 The most easily applied test is that due to Hehner, which is 

 best carried out as follows : The milk is diluted with an equal 

 volume of water, and a little 91 per cent, sulphuric acid run 

 in so that it forms a layer at the bottom. In the presence of 

 formaldehyde a violet-blue colour appears at the junction of 

 the two liquids, and the colour is permanent for two or three 

 days. This test will detect, easily, 1 part of formaldehyde in 

 200,000 of milk. Milk, in the absence of formaldehyde, gives a 



