52 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



water be placed in the 'other, the salt solution will soon be found to have 

 increased in volume and the water to have decreased. If the character 

 of the two liquids is then examined it will be found that the distilled 

 water is no longer pure, but that it contains salt ; while the saline solu- 

 tion will be no longer saturated, but of much less density. Salt has 

 therefore passed through to the water and water passed through to the 

 salt solution. There has been, however, as is evident, a difference in 

 the rapiditj^ with which the two substances have traversed the mem- 

 brane. That this process is not at ail analogous to filtration — in fact is 

 directly opposed to it — is seen in the continued ascent of the column of 

 liquid in one arm of the tube, showing that the water passes to the salt 

 solution even against a continually increasing hj'drostatic pressure. 

 The tendency is therefore for filtration currents to form in the opposite 

 direction to the osmotic current. If one liquid is water and the other 

 salt solution, the amount of water passing to the salt solution for each 

 equivalent of salt passing to the water is called the osmotic equivalent 

 of the salt. The osmotic equivalent is dependent upon the chemical 

 nature of the body and the concentration of its solution. 



Thus, for sodium chloride it is 4.3, meaning that for every gramme 

 of salt which passes through the membrane 4.3 grin, of water will pass 

 in the opposite direction to the salt. For sodium sulphate it is 11.6; 

 potassium sulphate, 12 ; magnesium sulphate, 11.1 ; alcohol, 4.2 ; sugar, 7.1. 



In general the osmotic equivalent increases with the temperature, 

 and varies very greatly with the concentration of the solutions. 



The rapidity with which different bodies diffuse through a porous 

 membrane is independent of their osmotic equivalent, but is directly 

 dependent upon their chemical nature and solubility, increasing with 

 solubility ; and bodies nearly related as to their chemical composition 

 are also nearly related as to rapidity of diffusion. The rapidity also 

 increases with increasing difference of concentration between the two 

 liquids and with increase of temperature. 



When two solutions of the same substance but of different densities 

 are allowed to diffuse into one another, the denser decreases in density 

 and the lighter increases, and the same alterations of volume occur as 

 would be the case were one of the liquids pure water. The osmotic 

 equivalent is in both cases the same, but in the former case the rapidity 

 of osmosis is much less. 



If two solutions of substances of different chemical composition are 

 allowed to diffuse, the rapidity will increase with increase in the chemical 

 affinity between the two substances. 



All colloids pass with difficulty through organic membranes, but as 

 they have a strong affinity for water they draw it with vigor through 

 organic membranes ; hence, their osmotic equivalent is very hisrh though 



