382 PROTEIN-COMPOUNDS. 



lime ; but unfortunately albuminate of soda (which, as we know, 

 does not coagulate on boiling) possesses this property in common 

 with casein. 



At an earlier period of organic chemistry, many other reactions 

 by which casein was characterised used to be described, as, 

 for instance, sulphurous acid, its difficult solubility in acetic 

 acid, &c. ; but all these means yield no definite result. More- 



ver, during the last few years, much attention has been devoted 

 to the behaviour of casein and of the protein-compounds gene- 

 rally with tests of the most varied kind; but however deserv- 

 ing of notice such endeavours may be, they have not produced 

 any great results, nor indeed could they be expected to do so, 



or independently of the fact that an endeavour to discover 

 any decisive reactions is mere groping in the dark, when the 

 investigation is not guided by one uniform leading idea, the results 

 of these experiments so frequently vary in their individual character 

 that it is often impossible to bring them into harmony. Any one 

 who has occupied himself with such investigations, and observed 

 the action of acids, bases, metallic salts, &c., under various rela- 

 tions, on the albuminous substances, can confirm the statement 

 that one and the same substance, under apparently similar rela- 

 tions, yields the greatest diversity of reactions, sometimes pre- 

 senting a similarity to one and sometimes to another protein- 

 compound. The various relations which modify these reactions, 

 and of whose nature we are still ignorant, render experiments per- 

 fectly useless, unless these circumstances be taken into account. 

 In general we may suspect the modifying influence, but in special 

 cases we are often quite in the dark. A very simple example will 

 illustrate our meaning. Casein is sometimes very readily soluble 

 in acetic acid, at other times it is rather difficult of solution, while 

 again there are other occasions in which it is almost insoluble in 

 that fluid ; we can only conjecture that the state of cohesion, the 

 earthy matters contained in it, &c., give rise to this difference; but 

 in individual cases it is often impossible to say which of these two 

 conditions, or whether any other, is influencing the result of the 

 special observation. 



I may in this place give another example of the difference 

 induced by inexplicable circumstances on reactions : on one occa- 

 sion a turbid acid solution of casein becomes perfectly clear on 

 the application of heat, on another the casein is entirely sepa- 

 rated on heating ; and thus acetic acid not unfrequently produces 

 only a slight precipitation in the milk of cows and other animals, 

 a true coagulum only separating on boiling. 



