of removing Matter from Solution in Water. 27 



phsenomenon ; he says that " at first sight it would appear not 

 a Uttle remarkable that the fresh water should regularly ebb 

 and flow with the tide. We must believe that the compressed 

 sand or porous coral rock acts like a sponge, and that the rain- 

 water which falls on the ground being specifically lighter than 

 salt, floats on its surface, and is subject to the same movements. 

 There can be no actual attraction between salt and fresh water, 

 and the spongy texture must tend to prevent all admixture 

 from slight disturbances. On the other hand, where the foun- 

 dation consists only of loose fragments, upon a well being dug 

 salt or brackish water enters, of which fact we saw an instance 

 on this small island.^^ 



Now I would not presume to deny the accuracy of this ex- 

 planation, still it is possible that the phsenomenon may be a 

 mixed one, and that the peculiar power which I have found to 

 be possessed by porous media may contribute towards the re- 

 sult by abstracting a portion (at least) of the salts from the sea- 

 water during its passage through the porous rock. 



Moreover, there are certain difficulties in Mr. Darwin^s ex- 

 planation. If the sand did not possess the power of removing 

 salts from solution, to a certain extent at all events, the rain- 

 water inside would tend, more or less rapidly, to become salt by 

 the commixture with it of the sea-water from without, which 

 would take place through the porous diaphragm so much the 

 more readily on account of their difference of density, in virtue 

 of the '^ osmotic force '^ so ably investigated by Professor Graham. 

 Indeed, the last fact mentioned by Mr. Darwin, viz. that when 

 the wall consists of loose fragments the wells are salt, tends to 

 show that it is only the very finely divided sand, the porous me- 

 dium offering a considerable extent of surface, which possesses 

 the power of removing the salts, for if the non-commixture of 

 the salt and fresh water arose only from their difference of 

 density, the coarseness of the diaphragm would not materially 

 affect it. 



As to the cause of this peculiar action, the only suggestion 

 which I may venture to offer is, that it possibly arises from the 

 weak chemical affinity in virtue of which salts are held in solu- 

 tion in water, being overcome by a physical force, the attraction 

 of cohesion, or more properly adhesion, exerted by the greatly 

 extended surface of the porous body ; this is not a solitary in- 

 stance of such weak chemical forces as solution being overcome 

 by a physical attraction. Gmelin* instances several other ex- 

 amples of the power of the physical force of '^ adhesion " to over- 

 come weak chemical attractions, e. g. : " When vinegar is filtered 

 through quartz sand, the first portion of liquid that runs through 



* Handbook of Chemistry, Cavendish Society's Translation, vol. i. p. 114. 



