Septembeb 21, 1906.] 



SCIENCE. 



361 



Cambridge Mr. Bedford reerected the ap- 

 paratus in another building. After having 

 surmounted great difficulties, he repeated 

 many of my experiments, and he informs 

 me that the numbers he has so far obtained 

 are in almost entire agreement with those 

 previously obtained by me. The molecular 

 depression in the case of cane sugar I found 

 to be 1.858, of potassium chloride 3.720, 

 and I understand that Mr. Bedford's ex- 

 periments agree with these results with a 

 discrepancy of less than 1 part in 1,000. 

 The most probable number obtained from 

 theoretical considerations would be in the 

 former case 1.857, in the latter 3.714. As 

 Mr. Whetham has pointed out, unless there 

 is some balancing of opposite errors of a 

 very improbable nature, it is difficult to 

 imagine a more direct vindication of the 

 application of thermodynamic considera- 

 tions to the phenomena of solution. I may 

 add that I also examined correspondingly 

 dilute solutions of sodium chloride, barium 

 chloride, sulphuric acid, potassium bichro- 

 mate, magnesium chloride and potassium 

 iodide; but, owing to the circumstances to 

 which I have referred, I was unable to 

 repeat these experiments in such a manner 

 as to enable me to attach great importance 

 to the resulting figures. Nevertheless, I 

 obtained values which strengthened the 

 conclusions to which I was led by the more 

 exhaustive examination of the dilute solu- 

 tions of sugar and potassium chloride. 



So far back as the Liverpool meeting of 

 this association I expressed a hope that the 

 experimental difficulties of the direct meas- 

 urement of osmotic pressures would be 

 overcome, as such direct measurement 

 would afford the most useful data by 

 means of which to obtain further light on 

 the much-vexed question of the nature of 

 solutions. I remember, also, that it was 

 the general opinion of those who had given 



attention to this matter that the experi- 

 mental difficulties were insuperable. 



I am glad, therefore, to have this oppor- 

 tunity of stating my high appreciation of 

 the manner in which Lord Berkeley and 

 Mr. Hartley have grappled with the diffi- 

 culties of this investigation. They have 

 proved that the osmotic pressure obtained 

 by direct measurement agrees with that de- 

 rived from vapor-pressure observations to 

 within less than 5 per cent.^ The agree- 

 ment is of great importance, as it dimin- 

 ishes our doubts as to the extent to which 

 the imperfections of semi-permeable mem- 

 branes may affect the validity of results 

 dependent upon their behavior, and points 

 to the possibility of determining the os- 

 motic pressures of concentrated solutions 

 by measurement of their vapor pressures. 

 I trust it will not be thought out of place 

 if I here refer to the interesting corre- 

 spondence which has recently appeared in 

 Nature on the thermodynamic theory of 

 osmotic pressure, and the allied, but by no 

 means identical, problem of the difference 

 between electrolytic and non-electrolytic 

 solutions. 



On the one side we have Professor Arm- 

 strong, whose chief desire appears to be 

 the vindication of the moral character of 

 what he terms 'the poor molecule'; and 

 Mr. Campbell, whose doubts concerning the 

 second law of thermodynamics are closely 

 connected with a lurking belief in the exist- 

 ence of Maxwell's 'sorting demons'; and 

 by way of reserves we have Professor 

 Kahlenberg, who contends that 'thermo- 

 dynamic reasoning can not be applied to 

 actual osmotic processes' on account of the 



O. p. de- 

 duced from 

 Direct O. P. Vapor Pres- 

 at 0° C. sure at 0° C. 

 69.4 



^ Concentration. 

 540 grains per lit. solution. . 67.5 

 660 grains per lit. solution. . 100.8 101.9 



750 grains per lit. solution. . 133.7 136.0 



Proc. Boy. Soc, June, 1906. 



