740 



SCIENCE 



[Vol. LVI, No. 1461 



if the temperature is not too ihigh while the 

 latter does not. The formation of a continuous 

 gell in the gelatin solution is preceded iby the 

 formation of suibmieroseopic aggregates which 

 occlude water and which are capable of swell- 

 ing and these aggregates or precursors of the 

 continuous gel increase in size and number on 

 standing. To test this idea the writer made 

 experiments with suspensions of powdered 

 gelatin in water and found that such suspen- 

 sions of powdered gelatin bad a much higher 

 wscosit}' than a freshly prepared solution of 

 gelatin. This was to be expected if the influ- 

 ence of acid on the viscosity of proteins is due 

 to the swelling of siibmicroscopic particles of 

 gel. It harmonizes with this fact that the vis- 

 cosity of solutions of crystalline egg albumin 

 is of a low order of magnitude, which was to 

 be expected, if solutions of crystalline egg 

 albumin contain few or no micellae. It was 

 found, moreover, that the viscosity of suspen- 

 sions of powdered gelatin increased under the 

 influence of acid or aUkali in the same way as 

 did the swelling of jellies or the osmotic pres- 

 sure of protein solutions. The viscosities were 

 measured at 20° C. When the suspension of 

 powdered gelatin was melted, it Avas found 

 upon rapid cooling to 20° C. that the viscosity 

 was considerably lower and that the influence 

 of acid had almost disappeared. By these and 

 a number of similar experiments it was pos- 

 siMe to prove that the similarity between the 

 influence of electrolytes on the viscosity of 

 gelatin solution and the influence of electiK)- 

 lytes on osmotic pressure is due to the fact 

 that the influence on viscosity in such cases is 

 in reality an influence on the swelling of sub- 

 microscopic protein particles. This proof was 

 made complete by showing that there exists a 

 Donnan equilibrimn between powdered parti- 

 cles of gelatin and a suiTounding weak gelatin 

 solution. 



VIII 

 It may Dot be amiss to illustrate by way of an 

 example why it is thait the neglect of measuring 

 the hj'drogen ion coneenbratioii of protein solu- 

 tions necessarily leads into errors. In a paper 

 published in 1921 by Kuhn,3 it was intended to 



3 Kuhn, A.: Kolloidcliem. Beihefte, 1921, xiv, 

 147. 



show that different aeids of the same valency 

 have different effects on the swelling of gelatin. 

 In order to furnisli such a proof it is necessary 

 to start with dsoeleetrie gelatin and to compare 

 the effect of different aeids on the swelling of 

 tJiis isoelectric gelatin at the same hydrogen ion 

 concentration of tlie gel, since only in that case 

 have the gels the same concentration of gelatia 

 ions. Instead of starting with isoelectric gelatin 

 or gelatin of a measured Pj^, Kuhn failed to 

 measure the p^j of his gelatin, though it makes 

 quite a difference whether aeid is added to iso- 

 electric gelatin or to gelatin at another p . 

 Further, instead of measuring the Pj, of the gel 

 Avitli the hydrogen eleotrode, Kuhn calculated the 

 hydrogen ion coneeaitrations from Kohlrausch's 

 tables as if acid had been added to water free 

 from gelatin and as if the presence of the protein 

 did not alter the hydrogen ion concentration. Our 

 titration curves, however, show that when aeid is. 

 added to isoelectric gelatin the hydrogen ion con- 

 centration is less than wlien acid is added to water 

 free from protein. And finally, on account of 

 the Donnan equilibrium the Pjj inside and outside 

 the gel are entirely different; yet no mention is. 

 made of the Donnan equilibrium in the paper 

 referred to. The hj^drogen ion concentrations of 

 protein solutions which were considered as equal 

 by Kuhn were on account of all these errors en- 

 tirely different, and it is quite natural that Kuhn 

 came to the conclusion that different monobasic 

 aeids have different effects on swelling, since it 

 would have been a miracle if with his faulty 

 methods he had ever compared two acids of the 

 same p ,. The same criticism applies to all the 

 older experiments on the influence of electrol;i'tes 

 on swelling in which the authors reached the con- 

 elusions that different anions lof the same valency 

 have different effects on swelling (Hofmeister 

 series). In all these experiments tihe authors 

 failed to measure the p^j of their gels and errone- 

 ously attributed effects due to differences of the 

 p of the gels to the difference in the nature 

 of the anion. 



IX 

 We therefore come to the eondlusion .that the 

 chemistry of proteins does not differ from the 

 chemistry of crystalloids, and that proteins 

 combine stoiehiometrically with acids and alka- 

 lies forming protein salts which dissociate elec- 

 trolytically. The enoi-mously large protein 

 dons and molecules can not dKfuse freely 

 through gels ' or many membranes which are 

 easily permeable to small crystalloidal ions. 



