THE THEORY OF OSMOTIC PRESSURES. 161 



spaces — a balance which is known to obtain under physio- 

 logical conditions. If we increase the volume of circulating 

 fluid we increase intracapillary pressure and the blood 

 volume tends to diminish in consequence of increased 

 transudation. If we diminish the capillary pressure by 

 bleeding the animal, absorption will predominate over exu- 

 dation, and the volume of circulating fluid will tend to 

 increase towards its normal amount. 



From this cursory study of some of the simplest examples 

 of transference of fluids and salts across membranes, we 

 may draw certain conclusions as to the main factors which 

 are of importance for the process. 



These are : (i) The permeability of the membrane to the 

 dissolved substances. This permeability may be of the 

 same character as the permeability of water, in which case 

 the rates of passage of the dissolved substances across the 

 membrane vary as their diffusibilities, and are therefore 

 probably some function of their molecular weights. On the 

 other hand the membrane may exhibit a certain attraction 

 for, or power of dissolving, some dissolved substances to the 

 exclusion of others, in which case there will be no relation 

 between the diffusibilities and rates of passage of the dis- 

 solved substances. 



(2) The osmotic pressure of the solutions. It is evident 

 that the rules deduced by Heidenhain from the accepted 

 theory of osmotic pressures, and quoted at the beginning of 

 this article, are fallacious in consequence of a too narrow con- 

 sideration of this second factor to the exclusion of the first. 

 At the same time it must be confessed that our knowledge 

 of the permeability of different membranes to different 

 substances, as well as of the factors on which this per- 

 meability depends, is still in an embryonic condition. 

 There can be no doubt that a careful exploration of this 

 field of research would yield results not only interesting 

 to the physicist, but also of incalculable value to the 

 physiologist in his investigation of the phenomena of 

 living things. 



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