428 



SCIENCE 



[N. S. Vol. XXXVII. No. 951 



It is well known that Pfeffer constructed 

 cells of clay whose pores were filled with a 

 precipitate of ferrocyanide of copper and 

 that the walls of these cells allowed water, 

 but not salts or sugars, to diffuse through. 

 When such a cell with semi-permeable 

 walls is filled with a salt or sugar solution 

 and put into a larger vessel containing 

 distilled water, the latter will diffuse into 

 the cell and raise the level of the liquid 

 until the hydrostatic pressure is equal to 

 the osmotic pressure of the solute. Such a 

 cell could therefore be used for the deter- 

 mination of the molecular weight of any 

 dissolved substance which can not pass 

 through its walls. 



Now, if it be true that living cells are 

 surrounded by semi-permeable walls and 

 that the osmotic pressure regulates the ex- 

 change of water between the cells (or tis- 

 sues) and the liquids of the animal body, 

 it should also be possible to use living cells 

 as osmometers for the determination of the 

 molecular weight of sugars or salts. 



The reader is aware that this possibility 

 had already been demonstrated for plant 

 cells (Tradescantia) by de Vries before 

 van 't Hoff had discovered the applicability 

 of Avogadro's law to liquids and that de 

 Vries 's observations induced van't Hoff to 

 interest himself in this problem. At the 

 time of de Vries 's work there was a differ- 

 ence of opinion concerning the molecular 

 structure of the sugar rafiinose and three 

 different formulse were offered, one giving 

 the sugar the molecular weight 396, the 

 second 594 and the third 1,188. 



By determining the concentrations of 

 cane sugar and raffinose which cause plas- 

 molysis, i. e., the shrinking of the proto- 

 plasm of the Tradescantia cells, de Vries 

 found that a 3.42 per cent, cane sugar solu- 

 tion and a 5.96 per cent, raffinose solution 

 had the same physiological effect. On the 

 basis of the assumption that solutions with 



an equal attraction for water have the 

 same number of molecules in equal vol- 

 umes, and since the molecular weight of 

 cane sugar is 342, the molecular weight of 

 raffinose should be (5.96/3.42) • 342 = 596. 

 In this way it was established by de Vries 

 that the molecular weight of raffinose was 

 594 and the formula CigHjoOio -f 5H,0 

 the correct one.- This formula for raffi- 

 nose is to-day accepted by the chemists. 



11. EXAMPLES FOE THE APPLICABILITY OP 



AVOGADEO'S LAW TO THE OSMOTIC 



BEHAVIOE OP ANIMAL TISSUES 



The oldest and, perhaps, best demonstra- 

 tion of the fact that the exchange of water 

 between animal cells and the surrounding 

 liquid is determined by Avogadro's law is 

 furnished by Hedin's experiments on red 

 blood corpuscles. His method consisted in 

 the determination of the changes of vol- 

 ume of red blood cells in various solutions. 

 The volume was determined with the cen- 

 trifuge. 



Hedin" first established the fact that the 

 blood cells do not change their volume if 

 put into solutions of the same molecular 

 concentration as the blood, no matter what 

 the nature of the solution, provided that 

 the substance does not enter the cell. 



VOLUME OF BLOOD CORPUSCLES IN 



.15m KNOj NaCl NaCHjCOO CaCL 



34.3 



34.4 



34.4 



34.3 



All the solutions had the same osmotic 

 pressure as a .15m KNO3 solution. The 

 agreement of the results is such that the 

 use of red blood corpuscles as osmometers 

 for the determination of the molecular 



' After Hoeber, Physical. Chemie d. Zelle u. 

 Gewebe, 1911, p. 86; De Vries, Jalirh. f. xoissensch. 

 Botanik., Vol. 14, p. 427, 1884. 



' Hedin, Sl-and. Arch. f. Physiologic, Vol. 5, 

 pp. 207 and 238, 1895. (The tables quoted here 

 are taken from Hamburger, ' ' Osmotiseher Druck 

 nud lonenlehre.") 



