SOLUTIONS 173 



ties of the different substances are in the proportion of their 

 molecular weights dissolved in equal volumes of the same solvent, 

 they exert the same osmotic pressure. This is parallel with the 

 law of Avogadro. The qualification here mentioned must, 

 however, be observed, that is, sugars, alcohols, and neutral 

 substances generally which are not electrolytes may be com- 

 pared together, while acids, bases, salts may be compared 

 together, but for reasons to be explained presently, the osmotic 

 pressures given by such substances, which conduct an electric 

 current, and are decomposed by it, are not comparable with 

 those of substances like sugar. Their osmotic pressures are in 

 general much greater. 



Pfeffer's method of experiment was based on the employment 

 of a membrane which allows water to pass through, but which 

 does not allow the dissolved substance, such as sugar, to pass. 

 If now we imagine a small vessel composed of this semipermeable 

 material, filled completely with a solution of sugar and con- 

 nected with a manometer, any change in volume of the liquid 

 will be indicated by the manometer. If the semipermeable 

 vessel is then immersed in pure water an increase of pressure 

 soon begins to be manifest by the movement of the mercury in 

 the manometer, and this is due to the passage of water from 

 without inwards through the membrane. The maximum 

 pressure indicated by the manometer is the osmotic pressure. 

 PfefTer's methods have been improved upon in more recent 

 years, and measurements have been made by Morse and Frazer 

 in America up to 20 atmospheres and more, also by means of 

 a special apparatus devised by Lord Berkeley and Mr. Hartley, 

 pressures have been recorded for strong sugar solutions up to 

 more than 100 atmospheres. 



The nature of the membranes which have been used is a matter 

 of interest on account of physiological considerations. The 

 material most commonly used in the earlier experiments was a 

 film of copper ferrocyanide, which is formed when a drop of 

 solution of copper sulphate is brought into contact with a 

 solution of potassium ferrocyanide, but measurement of pres- 

 sures could only be made possible when this material is deposited 

 in the pores of unglazed china-ware, of which the experimental 

 vessel to hold the solution is made. Through this material 

 water passes freely, but neither copper sulphate nor potassium 

 ferrocyanide, common sugar nor dextrose. Other artificial 



