174 CHEMICAL DISCOVERY AND INVENTION 



membranes have been prepared for use in the experimental 

 study of the phenomena, but there are many interesting cases of 

 natural membranes, of which one has been carefully studied by 

 Professor Adrian Brown within recent years. He finds that the 

 barley grain is covered with a membrane of this kind, which, so 

 long as the seed is uninjured, allows water from any solution in 

 which it is immersed to pass into the interior of the grain, but it 

 completely excludes sulphuric acid, common salt, and many other 

 substances. And even grains which have been boiled in water 

 so as to destroy their vitality retain this selective power, and 

 thus show that this power is due to a physical property of the 

 membrane and is not a physiological effect of living matter. The 

 importance of discoveries of this kind is obvious in connection 

 with the changes which go on in both vegetable and animal 

 tissues. There must be many different semipermeable membranes 

 existent in such tissues to account for the remarkable and rapid 

 exchanges between fluids contained in adjacent cells. To cite 

 one instance the ascent of the sap from the root to the stem and 

 often distant branches of a tree must be dependent on action of 

 this kind in which water passes freely, while the soluble contents 

 of the cells forming the wood and leaf are not suffered to be lost 

 or washed away by rain. 



If the principle is accepted that different substances taken in 

 the proportions of their molecular weights in equal volumes of 

 the solution have the same osmotic pressure, it is obvious that 

 by the determination of the osmotic pressure a method is pro- 

 vided for the determination of molecular weight. But the 

 experimental determination of osmotic pressure is not an easy 

 matter, and it is therefore more practicable to use for comparison 

 a solution of known osmotic pressure. Liquids which have the 

 same osmotic pressure are usually described as isotonic, and the 

 comparison may be made by observing whether or not water 

 passes from the solution to be tested into the standard solution 

 contained in a natural or artificial cell membrane. 



It is unnecessary in this place to pursue the subject further, 

 especially as it is treated very fully in all the best books on 

 physical chemistry. Enough, however, has been said to show 

 that the physics and chemistry of a cup of tea are more com- 

 plicated than is perhaps commonly supposed. 



The subject, however, is by no means exhausted. There is 

 reason to believe that in the act of dissolution in a liquid many 



