58 DIFFUSION AND OSMOTIC PRESSURE 



was drawn that the corpuscles had lost water, and hence that 

 the surrounding solution was of higher osmotic pressure 

 than the internal one. Several slight modifications of the 

 method were used, and many different solutions were com- 

 pared, the results being quite uniform with those obtained 

 by direct observation of the cells by De Vries and Hamburger. 



Still another manner of comparing osmotic pressures of 

 various solutions by means of plasmolytic phenomena in liv- 

 ing cells is that used by Wladimiroff, 1 who brought motile 

 bacteria into requisition for the purpose. He found that 

 these organisms cease to be motile when the osmotic pressure 

 of the surrounding fluid attained a certain magnitude. 

 Using as a criterion the degree of concentration at which 

 motion ceased, he compared the osmotic pressures of a num- 

 ber of solutions. His results are, in general, uniform with 

 those of the other authors just mentioned. The loss of 

 motion was due to extraction of water in a manner exactly 

 analogous to plasmolysis. 



In making turgor determinations by the plasmolytic 

 method the results may be given in various ways. The 

 usual method has been to give them in terms of a per cent, 

 solution of potassium nitrate, sometimes of sodium chlorid, 

 sometimes of sugar, etc. But with this method, whenever it 

 is desired to compare pressures which have been measured 

 by means of different plasmolyzing solutions, it becomes 

 necessary to make calculations which involve the molecular 

 weights of the substances used. A much better way to 

 express turgor pressures is in terms of fractions (e. g. , tenths) 

 of a molecular solution. But this, although it suffices for 

 non-electrolytes, fails utterly for electrolytes, because of the 

 unequal dissociation of different compounds. A y 1 ^- gram- 

 molecular solution of KNO 3 will give a much greater osmotic 



1 A. WLADIMIEOFF, " Osmotische Versuche an lebenden Bakterien," Zeitschr.f, 

 physik. Chem., Vol.VII (1891), pp. 529-43; also Zeitschr.f. Hygien, Vol. X (1891), pp. 

 89-110. 



