80 CASEIN. 



powder, but the latter is probably more convenient when 

 intended for distant destinations. 



The casein is prepared from milk in the usual manner, 

 by the addition of acid bodies, acetic acid, tartaric acid, etc., 

 the casein being precipitated along with a certain unavoid- 

 able small proportion of fat. Moreover, various acids are 

 used for precipitating casein from skim milk ; and this is 

 one reason for the irregularities in the quality of commercial 

 casein. On this point Bellamy reports that acetic acid, in 

 the form of vinegar, has hitherto been chiefly used, and 

 sulphuric acid occasionally. The casein must then be 

 neutralised before it can be used further, for which purpose 

 it is steeped in a solution of lime (lime-water) until it no 

 longer reddens blue litmus paper. This treatment furnishes 

 insoluble calcium acetate, which must be washed out before 

 the casein can be converted into glue. For this reason Bel- 

 lamy recommends that the casein should be thrown down by 

 means of vegetable substances, whose coagulant properties 

 are based on the presence of tannin, e.g., cutch, China bark, 

 oak bark and other barks, sumach, etc. Special success has 

 attended the use of wild-cherry bark (prunus virginiana) for 

 this purpose. The resulting casein does not require neutral- 

 ising, and is easily worked up, besides being more uniform 

 in quality. At the same time, the quantity of the precipitant 

 is immaterial, since any excess remains in solution and 

 therefore does not stick to the casein when this is boiled. 

 Whatever method of preparation is adopted, it is always 

 advisable to add to the casein a little sodium arsenate, 

 Na. 2 HAsO 4 , or some other salt of arsenic, since this increases 

 the adhesive power and forms a kind of chemical mordant. 



Another method of preparing casein specially for use as 

 an adhesive is the following. Milk is left to stand in a cool 

 place and the cream carefully skimmed off, this being re- 

 peated as long as any cream continues to rise, the milk being 



