PRECIPITATION AND COAGULATION 127 



Ca > Sr > Ba while trivalent metals (Al) act still more ener- 

 getically. In opposition to this view Donnan (11) (12) (41) (48) 

 (13) advances the hypothesis that the blue color of concen- 

 trated solutions of cobalt chloride is due to the formation of 

 complex ions of the type CoCl 2 .Cl 2 ". Lewis points out, however, 

 that this view is inconsistent with the fact that it is possible to 

 change the color of the solution from blue to red by mere dilu- 

 tion, without altering the active mass of any component of the 

 system except water. Other objections against Donnan's view 

 have been urged by Hartley and H. C. Jones. 



The peculiar interest to the biological chemist of the possi- 

 bility thus indicated, that substances dissolved in water form 

 loose combinations with the solvent, lies in the especial signifi- 

 cance of water in relation to the protein and polypeptid struc- 

 ture. As indicated in the first chapter, dehydration of a protein 

 may result in the following reactions: 



/NH 3 OH HOOC. /NH 3 .OOC.R,NH 3 OH 



R; + / R = R v + H 2 



N COOH HOH 3 N X N COOH 



, NH 3 OOC.R.NH 3 OH , NH.OC.R.NH 3 OH 

 R' = R( + H 2 



N COOH X COOH 



, NH.OC.R.NH 3 OH , NH.OC.R.NH 2 

 R' = R' + H 2 



X COOH X COOH 



, NH.OC.R.NH 2 / NH.OC.R.NH 

 R' = R' I +H 2 



X COOH X CO- 



and hydration, of course, may result in the reversion of this 

 series of changes. 



That proteins may be thrown out of solution in two very dif- 

 ferent conditions of hydration is evident from the researches 

 cited in the previous sections of this chapter; it is even more 

 clearly shown by the following experiments (67): 



Anhydrous casein dissolves readily in cold anhydrous * formic 

 acid; still more readily in hot formic acid If, to a two per cent 

 solution of casein in formic acid, we add a fairly concentrated 



* Anhydrous, that is, save for traces of moisture derived from the atmos- 

 phere. 



