36 CHEMICAL STATICS 



It is clear, therefore, that proteins of this class must be pre- 

 cipitable only within a very narrow range of hydrogen- and 

 hydroxyl-ion concentrations and that the probability of simul- 

 taneous precipitation of more than one protein is very slight 

 unless the proteins chance to be acids or bases of very nearly 

 equal strength. In very few cases, however, do we possess, at 

 present, any very reliable guarantee that this does not occur, 

 and in a few instances, at all events, we are positive that it does 

 occur. For example, the various members of the paranuclein 

 series, which are derived from casein by its partial hydrolysis, 

 are all precipitable by acetic acid, and, some of them at least, 

 are not redissolved by the addition of moderate excess of acetic 

 acid. True, there are recognizable differences between the basic 

 functions of the various paranucleins and casein. The para- 

 nucleins are precipitated in solutions of lower acidity than casein, 

 and doubtless an acid could be found the solution of which, at 

 a concentration sufficiently acid to precipitate paranuclein, would 

 not be sufficiently acid to precipitate casein. But this acid has 

 not, as yet, been discovered and so a reliable method for sepa- 

 rating casein from paranuclein in a solution containing a mixture 

 of these proteins is yet to seek; while the separation of the dif- 

 ferent members of the paranuclein group from one another is a 

 task replete with difficulties and uncertainties.* 



It is not intended to imply, by the above remarks, that the 

 isolation of free casein is as a rule attended with danger of con- 

 tamination with paranuclein, because, fortunately, the presence 

 of paranuclein in solutions of casein is not to be feared unless the 

 casein has undergone appreciable hydrolysis in an acid medium. 

 I am merely pointing out the imaginary difficulties which would 

 be encountered in endeavoring to separate casein from para- 

 nuclein, were they found in nature in the same fluid, as an illus- 

 tration of the very real difficulties which unquestionably attend 

 the separation and isolation of other proteins which are insoluble 

 in distilled water when uncombined with acids or bases. 



All that can be said, therefore, when a protein isolated by means 

 of a certain procedure, is found repeatedly to yield the same 

 physical, physico-chemical and chemical constants, is that it is 

 a protein or mixture of proteins of constant composition provided 

 that particular method of isolation be employed and no other. 

 * Vide T. Brailsford Robertson (31). 



