543 SCIENCE PROGRESS 



a matter of very great importance both in order to determine 

 the actual magnitudes of the pressures and in order to provide 

 data for a theory of solutions which shall be applicable under 

 conditions other than those of " infinite dilution." 



It may be asserted emphatically that nothing, at the present 

 time, can take the place of direct measurements of osmotic 

 pressure carried out with the greatest care and exactitude. 

 Calculation fails utterly to represent the observations that have 

 been made : attempts to substitute indirect measurements for 

 direct measurements are almost equally useless : firstly, because 

 calculations are required which often involve approximations or 

 the use of constants of doubtful accuracy ; secondly, because it 

 is impossible to make isothermal measurements of the freezing- 

 point or boiling-point of a series of solutions, whilst vapour- 

 pressure measurements although made isothermally are usually 

 far from exact. 



The foregoing statement will serve to explain the great 

 interest and importance which attaches to the exact measure- 

 ments of osmotic pressure which have been made during the 

 opening years of the present century by the Earl of Berkeley 

 and his colleagues in England and by Prof. H. N. Morse and his 

 colleagues in America. The American work, in its general 

 features, follows the methods used a quarter of a century before 

 by Pfeffer and will be described as a sequel to his work ; but ten 

 years of laborious experiment were required before all the main 

 sources of error were eliminated : the measurements extend 

 from decinormal to normal concentrations, whilst the range 

 of pressures is from 2 to 25 atmospheres and the range of 

 temperatures from o° to 8o° C. The measurements of the Earl 

 of Berkeley and Mr. E. G. J. Hartley, which extended the range 

 of pressures up to 135 atmospheres, were carried out with a novel 

 type of apparatus, which will be described most conveniently in 

 the later part of the present article. 



C. Pfeffer's Experiments 



The experiments described in Pfeffer's Osmotische Unter- 

 suchungen (Leipzig, 1877) cover a very wide range of 

 phenomena. Observations were made of osmosis through 

 membranes of many kinds ; some of them were permeable 

 to the solute as well as to the solvent, others were permeable to 

 the solvent only. Experiments were made both on the rate of 



