MILK PRESERVATIVES BUTTER. 357 



with dilute sulphuric acid, and extracted with dilute alcohol. 

 The alcoholic solution is neutralised with lime water and 

 evaporated to small volume ; the residue is then slightly 

 acidified with dilute sulphuric acid and extracted with ether. 

 The ethereal solution is then evaporated, when the benzoic 

 acid is left and can be recognised by its odour. 



4. Formaldehyde in the form of " Formalin," which is a 

 40% solution in water of the real formaldehyde, = CH 2 , is 

 a very efficient preservative. It can be detected by Ilehner's 

 test, which consists in diluting the milk with an equal volume 

 of water, placing the- mixture in a test tube, and pouring in a 

 little sulphuric acid (specific gravity about 1/825) to which a 

 drop of ferric chloride or other oxidising agent has been added, 

 down the sides of the tube, so as to form a layer at the bottom. 

 If formaldehyde be present a violet or blue colour occurs at 

 the point of contact of the two layers. In this reaction 

 Hehner* finds that the casein of the milk takes part ; it can- 

 not, therefore, be obtained with aqueous solutions of formalde- 

 hyde. 



5. Fluorides or Fluosilicates are also possessed of good 

 antiseptic properties. They can be detected, if present, in the 

 ash of the milk by the usual reaction for hydrofluoric acid 

 its etching effect on glass when it is liberated by the action of 

 strong sulphuric acid. 



BUTTER. With genuine butter, the chief differences in 

 chemical composition are in the proportions of water, salt, and 

 casein. These are determined by drying a weighed quantity 

 in a flat-bottomed dish at 100 until it ceases to lose weight; 

 the loss gives the water present. The residue is then extracted 

 repeatedly with ether, the insoluble matter dried and weighed. 

 The weight gives the amount of casein and salt. The residue 

 is then treated with hot water, filtered, and the amount of 

 chlorine determined by titration with standard silver nitrate, 

 using potassium chromate as indicator, in the usual way. 



The most difficult part of such analytical processes is the 

 taking of a satisfactory sample ; the amount of water present 

 usually varies very much in different parts of the same mass 



* Analyst 181)6, 92 ; J.C.S. 1896, abst. ii. 583. 



