586 



SCIENCE 



[N. S. Vol. 2XXIV. No. 879 



solution, and if this dish is put into water, 

 the dissolved particles, in their futile ef- 

 forts to diffuse into the surrounding solu- 

 tion, will cause a pressure. This pressure, 

 which is perfectly analogous to the tension 

 of a gas, Van't Hoif called osmotic pres- 

 sure. According to Van 't Hoff every mole- 

 cule exerts the same pressure; in other 

 words, solutions of an equal molecular con- 

 centration have the same osmotic pressure. 

 This was also proved by experiment, hut 

 only for substances belonging to the same 

 category. The values of osmotic pressure 

 of substances belonging to different cate- 

 gories were compared, and showed consid- 

 erable differences. E. g., equimolecular 

 solutions of sugar and salt showed an alto- 

 gether different osmotic pressure. That of 

 the NaCl solution was 1\ times higher 

 than an equimolecular solution of sugar. 



This was the situation in 1885, when 

 Van't Hoff published his theory. No 

 wonder, therefore, that the theory could 

 not be generally accepted. 



It fell to Arrhenius's theory of electro- 

 lytic dissociation to explain away" the diffi- 

 culty contained in Van't Hoff's theory. 

 According to the Swedish investigator the 

 salts in a watery solution, unlike sugar, 

 dissociate partly into ions. To this Van't 

 Hoff added the idea that each ion exerts 

 the same osmotic pressure as would an un- 

 dissoeiated molecule.^" Accordingly the 



' Svante Arrlienius, ' ' Ueber die Dissociation der 

 in Wasser gelosten Stoffe," Zeitschr. f. PhysiJc. 

 Chemie, I., 630, 1887. Cf. also, Arrhenius, Behang 

 till Kongl. Svenska Vet. Akad. Handlingaar, 8, 

 No. 13 and 14, 1884. 



" Van 't Hoff, ' ' Die Eolle des osmotischen 

 Druelies in der Analogic zwischen Losungen und 

 Gasen," Zeitschr. f. Physilc. Chemie, I., 481, 1887. 

 He says here: "... Thus it may appear that to 

 claim Avogadro's law for solutions as forcibly as 

 I have done it here is rather unwarranted. How- 

 ever, my decision in this matter I owe to Arrhenius, 

 who in a letter calls my attention to the proba- 

 bility that in the case of salt solution and the liie 

 we have to deal with a dissociation of ions." 



number of particles causing osmotic pres- 

 sure is much larger in a salt solution than 

 in an equimolecular sugar solution ; in the 

 above mentioned case it was 1^ times as 

 large. This explanation removed the ob- 

 stacles of Van't Hoff's theory and the 

 isotonic coefficients of de Vries had now a 

 clear meaning. 



It was now obvious that if the coefficients 

 of NaCl and of sugar were 3 and 2, re- 

 spectively, this was due to the fact that 

 through the partial dissociation of NaCl 

 into the ions Na and CI, the number of 

 water-attracting particles became 1^ times 

 larger than that of the sugar solution. 



It is then not only to de Vries and Van't 

 Hoff, but also to the great Swedish genius, 

 that we owe a heavy debt of gratitude. 

 This not only because the theory of elec- 

 trolytic dissociation forms a necessary 

 supplement to Van't Hoff's original the- 

 ory, but also because it has itself "become 

 of tremendous importance to the medical 

 sciences. 



Without exaggeration, I think, we can 

 apply what Wilhelm Ostwald said in this 

 connection about chemistry, to the medical 

 sciences: "Seldom has a lucky thought 

 thrown so much light on so many and so 

 difficult problems"; and in 1890, i. e., 

 three years after the theory of osmotic 

 pressure has been known in its perfect 

 form, Van't Hoff says of the theory of 

 Arrhenius that "it has almost become a 

 fact. ' ' 



Quite inestimable has been the influence 

 exerted by the de Vries- Van't Hoff -Ar- 

 rhenius theory on our sciences. There is 

 hardly a chapter in physiology that would 

 not bear signs of this influence. 



Nowhere has the application of the com- 

 bined theory of the three great men been 

 so intensive as in the physiology of the 

 blood. This is easy to understand if we 

 consider the fact that the blood corpuscles 

 unlike most of the other cells can be kept 



