54 CASEIN. 



in the form of powders or paste, which only require thinning 

 out with water to make them suitable for use, and many of 

 these are characterised by the fact that they will stand a 

 reasonable amount of washing or rubbing. Distemper paint- 

 ing, or the use of various pigments mixed with water, has 

 long been known ; but applied in this simple way, the pigments 

 were loose and rubbed off badly, and to fix them on the 

 surface it has long been customary to add size or glue. But 

 the development in the dairy industry, and the desire to 

 utilise all the constituents of milk, has led to the production 

 of casein in fair quantities, and its employment as a fixing 

 agent to distemper paints. 



Casein belongs to the group of animal albumens, compounds 

 of carbon, hydrogen, oxygen, nitrogen, and sulphur, of all of 

 which it may be said that comparatively little is known of 

 their exact chemical composition. There are a number of 

 these bodies known, differing from one another in their 

 general properties. White of egg, gelatine (the basis of glue), 

 blood albumen, casein of milk, are examples ; they have been 

 the object of much investigation at the hands of chemists, 

 although very little that is absolutely certain is known of 

 them. In milk there exists, first, the fat which forms the 

 basis of butter, then a small quantity of sugar, and, in solution, 

 a nitrogenous body to which the name of casein is given. This 

 body may be precipitated out by the addition of a little acid 

 or by some metallic salt. When the precipitation is carried 

 out by means of rennet, on ordinary milk, all the casein and 

 fat are thrown out together, and the product pressed forms 

 the foodstuff, cheese. If, however, by the use of separating 

 machines the fat is first extracted as butter, then on precipita- 

 tion almost pure casein is thrown out. It may be noted here 

 that the action of rennet is somewhat different to that of 

 acids, inasmuch as the former does not completely throw 

 down the casein, but leaves some amount in solution. It is 



