.ioiin William DRAPEB. 



booic acid under one of 10 atmospheres, mid hydrogen sulphide 

 under n pressure of 241 atmospheres. Since the force impelling 

 the particles of one gas into the interstices of another, without a 

 septum, never exceeds the pressure of one atmosphere, it is plain 

 that the source of this remarkable power must lie in the membrane 

 itself. So that the caoutchouc membrane must, in this experiment, 

 have condensed the sulphurous-acid gas and the hydrogen-sulphide 

 gas into liquids, which were then, in this condition, transferred 

 through the film and evaporated on the other side precisely as with 

 the water film. 



With regard to liquid osmose Dr. Draper shows that the only 

 essentials are : First, that both the liquids should wet the barrier ; 

 second, that they should rise to different heights in tubes made of it, 

 and, third, should be capable of uniting chemically with each other. 

 All solid bodies which act as barriers have pores, which, while too 

 small to permit leakage, yet allow interchange of indefinitely small 

 columns of liquid. Such are plates of Villarica porcelain clay, of 

 Brazil indurated steatite, and of certain varieties of compact sand- 

 stone. No liquid can pass a barrier the pores of which it cannot 

 wet; but, since water by electrifying it positively may be made to 

 wet mercury, electricity modifies osmose. The separation of water 

 from litmus through a membrane having alcohol on its other side, 

 he concludes, is " only a refined kind of filtration, which, probably, 

 may hereafter become of considerable importance in its applications 

 in the arts, as in the separation of coloring matter from solutions, 

 or the preparation of medicines, such as the vegetable alkalies, which 

 should be formed from colorless solutions." 



In a paper published in 1846 Dr. Draper applies these principles 

 very ingeniously to explain the circulation of the sap in plants and 

 the blood in animals. Both these motions, he says, depend on the 

 following simple physical principle: "That if two liquids communi- 

 cate with one another in a capillary tube or in a porous or paren- 

 chymatous structure and have for that tube or structure different 

 chemical affinities movement will ensue ; that liquid which has the 

 most energetic affinity will move with the greatest velocity and may 

 even drive the other fluid entirely before it." In the rootlet the 

 phenomenon is simply an osmotic one, a flow taking place from the 

 water without into the mucilaginous sap within, precisely as water 

 flows into gum-water through a bladder. In the leaf the weak 

 ascending watery sap rises to its upper surface and there obtains 



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