128 



SCIENCE. 



[N. S. Vol. XIV. No. 343. 



There are two ways along whicli a reply 

 to sucli a question can be given. I might 

 trace in outline the general conclusions, 

 and might speak to you of the laws of 

 chemical change, of reaction velocity, of 

 electro-chemical processes, but I should not 

 be able to do much without the use of 

 a rather complicated formula, which the 

 character of this introduction excludes. 



The second way is of a special nature, 

 and I will venture on it, to trace on what 

 lines physical chemistry wrought out its 

 conclusions and what success it met with. 

 The particulars will apply to one of the 

 best known and farthest reaching achieve- 

 ments, to the investigations on osmotic 

 pressure. 



Let me begin with that particular attrac- 

 tion for water which we find in some sub- 

 stances very familiar to us, say in ordinary 

 burnt lime. If the lime is kept in a well- 

 filled flask or bottle, which is not hermet- 

 ically sealed, water from the surrounding 

 air will be attracted by the lime. This will 

 augment the volume of the lime, and the 

 flask or bottle will eventually give away ; 

 a tremendous force is thus developed, too 

 large, perhaps, to be exactly measured. 



But, on a smaller scale, we may follow 

 up quantitatively the analogous process 

 with sugar, for instance, in a dilute solu- 

 tion, say a 1-per-cent. solution. This will 

 attract water also, as may be shown by 

 filling with the solution a flask, porous, but 

 permeable for water only, and by placing 

 this flask, when well sealed, in water. 

 Then water will enter it till, if the flask 

 holds, a pressure of two-thirds of an 

 atmosphere is attained, as was measured 

 by Pfefier. ^ 



"We may generalize and say every solu- 

 tion has the tendency to diff"use into the 

 solvent as if it were attracted by the 

 solvent, and this tendency will produce a 

 pressure if the difi'usion is prevented by a 

 membrane. This pressure, for more than 



a century studied as osmotic pressure, has 

 a well-defined amount ; it was known to 

 vary with concentration, with temperature, 

 and with the nature of the substance dis- 

 solved, etc, , and this was all we knew about 

 it until the way in which physical chemistry 

 worked was applied to it. The result was 

 so transparent that every student may now 

 calculate readily for any dilute solution 

 what its osmotic pressure is ; for all may be 

 summed up in this one expression : 



P=0.08 CT, 



with P, the osmotic pressure in atmospheres, 

 T, the absolute temperatures, 0, the con- 

 centration or number of gram molecules of 

 dissolved substance in one liter of the solu- 

 tion. The above value of the pressure for 

 a 1-per-cent. sugar solution is, at once, got 

 at by this formula. Let me only insist on 

 the different way in which physical chem- 

 istry works as compared with stereochem- 

 istry. Physical chemistry does not seek 

 the solution of problems by trying to reveal 

 the constitution of matter, but it works out 

 between measurable things relations to 

 which the calculus may be applied. 



This is not all. Looking upon the tre- 

 mendous work which atomic chemistry has 

 achieved, one must acknowledge that in re- 

 search relatively little up to the present 

 has proved of value as to what most in- 

 terests us, the problem of life. Quite the 

 opposite can be maintained with the lines 

 followed up in physical chemistry ; and 

 even ten years ago, I used an occasion like 

 this at Utrecht to point out the large part 

 which this osmotic pressure, the laws of 

 which physical chemistry revealed, plays 

 in physiology. 



I could indicate the result of many a 

 physiological investigation, pointing to the 

 fact that osmotic pressure is a fundamental 

 factor in the most difierent vital functions 

 in plant and animal existence. According 

 to de Vries, it regulates the growth of the 



