318 CHEMISTRY OF THE PROTEIDS CHAP. 



added. An acid solution of albumin which contains salts coagulates as 

 soon as it is boiled. If the reaction be alkaline, precipitation is never 

 so complete as with acid solutions, but an alkaline solution containing 

 salts, especially a calcium salt, always becomes turbid, and is partially 

 precipitated on being heated. 



The following points are also of interest : Any albumin solution, 

 containing salts, acids, or bases, becomes more alkaline on being 

 coagulated ; therefore a solution which, to begin with, is neutral 

 or even feebly acid becomes distinctly alkaline on boiling. Why 

 this happens is not known definitely, but the author, who has 

 discussed heat coagulation at greater length in his Physiological 

 Histology, 1902, pp. 58-68, has arrived at the conclusion "that any 

 factor which tends to prevent the formation of hydrogen-ions will also 

 prevent coagulation." " By regarding proteids as hydrogen salts we 

 may assume that the hydroxyl-radical of alkalies prevents coagulation 

 either by preventing the intramolecular change" (i.e. conversion of 

 pseudo-bases into real bases and pseudo-acids into real acids see this 

 book, p. 219) "which occurs normally when heating neutral or acid 

 proteid solutions, or, if the intramolecular change does take place, by 

 neutralising the acid hydrogen-ion which is liberated." The increase 

 in alkalinity, which ^s produced by heating albumin solutions, is 

 explained thus : " If the pseudo-acid radical l of the proteid is con- 

 verted into a real acid, it will change the pseudo-basic radical into a 

 real base, which latter, by splitting off ammonia, could produce the 

 alkaline reaction." The view of the author is not only supported by 

 Schadee van der Does' 2 observation that addition of silver oxide, 

 Ag 0, prevents heat coagulation (see index), as does also the addi- 

 tion of osmium tetroxide, according to Monckeberg and Bethe, 3j 

 but also by the recent investigations of Heffter, 4 who showed that a 

 potential hydrogen-ion must exist in the albumin molecule, which, by 

 uniting with sulphur, gives rise to sulphuretted hydrogen (see p. 97). 

 That albuminous compounds are plurivalent acids and bases is 

 referred to on p. 218, and on the author's theory it is quite conceiv- 

 able that, if not all, at least some of the amino-acid side-chains in the 

 albumin-molecule, having become hydrolysed by heat, will then act on 

 the remainder of the molecule as would any free acid. Heat coagula- 

 tion is therefore brought about by one portion of the albumin molecule 



1 By an oversight the word pseudo-basic has been substituted for pseudo-acid in the 

 original. 



2 Schadee van der Does, Zeit. f. physiol. Chem. 24. 351 (1897). 



3 Monckeberg and Bethe, Arch.f. mikr. Anat. 54. 135 (1899). 



4 Heffter and Hausmann, Hofmeisters Beitrdge, 5. 214 (1904). 



