96 



The Origin of Osmotic Effects. 



considered from this point of view in our previous " Studies " (VII I, 

 pp. 111*112 ; X, p. 130). 



The exceptional rapidity, to which Prof. Brown directs attention, with 

 which ethylic acetate acts in promoting the entry of water into the grain is 

 also easily explicable from the same point of view. Entering together with 

 water, it should render water within the grain more active and more attractive 

 of external water (by promoting its dissociation, (HaO)* — * #H 2 0) than the 

 water would be which entered alone from a solution of an indiffusible solute, 

 as in such water (on account of its homogeneity) the osmotic stress would be 

 at a minimum.* 



It is obvious that the argument now put forward may be applied to the 



discussion of a great number of more or less obscure physiological phenomena. 



It may be desirable to consider the rise of the sap in trees from such a point 



of view. The argument affords an explanation of the well-known efficacy, 



for example, of mercury salts, of iodine and of alkaloids as drugs. It should 



point the way to the production of medicaments adjusted to their purpose — 



according as it is desired that they should penetrate this or that membrane. 



It may lead to the discovery of a method of using stains as the means of 



determining whether this or that membrane or layer in a cellular tissue is 



to be regarded as a mere sieve or as differentially penetrable, inasmuch as 



stains — whieh hitherto have been used all but empirically — must vary greatly 



in penetrative power and it should be possible to grade them, according to 



their diffusibility, by observations similar to those made by Prof. Brown. 



* [February 15, 1909. — Attention has been specially drawn in No. VIII of our Studies 

 (p. 108) to the behaviour of methylic acetate as a weak hydrolyte in comparison with the 

 strong hydrolyte cane-sugar ; the observations now under discussion appear to afford 

 complete confirmation of the argument there put forward that in discussing the phenomena 

 of hydrolysis it is necessary to take into account not only the condition of the medium 

 but also the nature both of hydrolyte and of hydrolyst, which are reciprocally concerned 

 in the change. The argument should be extended to colloid and other surfaces. Sir 

 James Dewar has shown that solids differ greatly in their power of attracting and holding 

 gases at low temperatures ; hydrolytes and dissolved substances generally, we must 

 suppose, also differ in the extent to which they undergo " hydration " ; wetted surfaces 

 generally must also differ in the extent to which they become hydrolated ; consequently, 

 it is to be supposed that more or less considerable variations will be met with when 

 differential septa are studied comparatively. Apparently the barley septum is not 

 penetrated even by ammonium chloride, so that it is more exclusive than that of red 

 blood cells, which are rapidly penetrated by this salt but scarcely if at all by ammonium 

 sulphate. The difference between ammonium chloride and ammonia is very striking, the 

 latter resembling ethylic acetate in passing rapidly into the seed and in promoting the 

 ingress of water ; this behaviour is easily understood, as it exists in solution partly in 

 the free state and partly, it may be supposed, as the hy drone H 3 N : OH 2 , the hydroxide 

 being present in only very small proportion. If ammonia were contained in solution as 

 the hydroxide, its behaviour would undoubtedly be that of caustic soda.] 



