AS INDICATED BY OSMOMETERS. 57 



ARRANGEMENT OF THE OSMOMETERS TO OPERATE AGAINST WATER. 



AND THEIR STANDARDIZATION. 



In the experiments so far carried out, the osmometers have usually 

 contained a 5-weight-molecular^ solution of sucrose in water, and only 

 experiments so performed will be here presented. What may be the 

 order of magnitude of the water-absorbing force effective at the periph- 

 eries of plant roots we have as yet no means of knowing, but it is surely 

 safe to suppose that the intensity of the osmotic force employed in our 

 osmometers (that of a 5-weight-molecular solution of cane-sugar) very 

 far exceeds that of the corresponding force usually occurring at active 

 plant surfaces. If such studies as the present ones are carried farther 

 it will be desirable to employ lower concentrations, but to accomplish 

 that it will be necessary to improve the apparatus so as to allow the 

 determination of smaller absoi-ption increments than could be read with 

 our instruments. So concentrated a solution was used simply to pro- 

 duce relatively high rates, thus allowing the use of our crude apparatus 

 and avoiding further delay in the preliminary attack upon the important 

 problem before us. It was our idea that the principles involved may 

 probably be much the same when a force of great magnitude is em- 

 ployed as when one of less intensity produces the water movement. 



Preliminary tests showed that the passage of cane-sugar through our 

 membranes was practically negligible. Not so unimportant, but still 

 probably negligible here, is the apparent hydrolysis of cane-sugar in 

 aqueous solution ; no matter how pure may be the sucrose used, there 

 seems always to be some glucose present, and the amount of this seems 

 to increase (as would be expected) with the age of the solution. This 

 matter, though investigated to some extent, does not require attention 

 in this publication; it was found that a good quality of "granulated 

 sugar" furnished as satisfactory and consistent results in our work as 

 did thoroughly washed *'rock candy." On this account the former 

 variety of cane-sugar was used in the tests about to be reported. 

 Freshly made solutions were always employed. 



^This means 6 gram-molecules of cane-sugar dissolved in 1,000 grams of water, following the 

 elaborate and beautiful work of Morse and his colleagues on the osmotic pressures obtainable 

 with cane-sugar solutions and membranes impermeable to this solute, as related to the concen- 

 tration of the solution and to the temperature. This work has indicated that the diffusion 

 tension of the solute, or the attraction of the solution for water (the osmotic pressure of the solu- 

 tion, as it is usually termed — though osmotic pressure is as much a function of the membrane 

 employed as it is of the diffusion tension of the solute) is proportional to the so-called molar 

 fraction, the ratio of the number of molecules of solute to that of solvent. In this connection, 

 see Morse, H. N.. W. W. HoUand, E. G. Zies, C. N. Myers, W. M. Clark, and E. E. Gill. 

 The relation of osmotic pressure to temperature. Part V. The measurements. Amer. Chem. 

 Jour. 45: 554-603. 1911. Also see: Earl of Berkeley and E. G. J. Hartley. On the osmotic 

 pressure of some concentrated aqueous solutions. Trans. Roy. Soc. London 206A: 481-507. 

 1906. The whole matter is briefly but rather clearly discussed by Findlay: Findlay, A., Osmotic 

 Pressure. London, 1913. [Pages 1-84.] The crying need for an appreciation by biologists of 

 this important advance in physical chemistry has been well emphasized by Renner: Renner, O., 

 Ueber die Berechnung des osmotischen Druckes. Biol. Centralbl. 32: 486-504. 1912. 



