286 SCIENCE PROGRESS. 



it to a vital action of the red blood -corpuscles, which 

 merely means that the process is at present inexplicable. 



Since the publication of the above paper, Hamburger 

 has published a number of others on the relationships 

 between the blood-corpuscles and plasma, and between the 

 circulating blood and the extravascular lymph. To some 

 of these we shall have occasion to refer later on. 



In the meantime publications of Van t' Hoff and of 

 Ostwald have given the impetus to a number of other 

 researches by physiologists on the bearing of osmotic 

 pressures on physiological processes. These researches 

 fall naturally into two main groups. 



In the first place, just as we can estimate the work done 

 by a muscle by measuring the load raised and the height 

 to which it is raised during a given time, so we can deter- 

 mine the minimum work done by a secreting cell by 

 measuring the osmotic difference between the secreted fluid 

 and the fluid from which it is formed, i.e., the blood- 

 plasma. Moreover, in many cases we can use a deter- 

 mination of the osmotic pressures in order to judge whether 

 the absorption of a fluid across a given membrane is a 

 simple physical process, dependent on osmotic differences, 

 or whether the fluid moves against pressure, and the 

 absorption must therefore involve the active intervention 

 of the cells forming the membrane. Of works of this class, 

 perhaps one of the most interesting is that by Dreser l on 

 the work done in the secretion of urine. 



Dreser makes use of the depression of freezing point 

 to determine the osmotic pressures of blood or blood- 

 plasma and urine. Since blood-corpuscles are destroyed 

 in solutions of urea, they cannot be used as an index to 

 the tonicity of this fluid. He points out in the first place 

 that, from Van t' Hoffs results, it is possible to calculate 

 directly the work done in concentrating a solution by the 

 separation of a certain amount of pure water in the form of 

 ice. If A is the depression of freezing point and T the 

 absolute freezing point of the solvent (i.e., for water, 273 , 



1 Ueb. Diurese und ihre Beeinflussung durch pharmakologische Mittel, 

 Arch./. Exp. Path., xxix., p. 307, 1892. 



