OSMOSIS AND ABSORPTION BY ROOTS. 53 



two salts, for it separates tlie one frc^in tlie other, and for 

 every one equivalent of potassium which it absorbs 

 leaves behind in the water more than thirty equivalents of 

 sodium. Manganese and iron, iodine and chlorine, are 

 likewise isomorphous bodies ; yet the iodine plant sepa- 

 rates one equivalent of iodine in sea-water from many 

 thousand equivalents of chlorine. 



The known laws of osmosis, and of the diffusion or 

 interchange of water and salts through a dead membrane 

 or a porous mineral body, give no explanation whatever 

 of the action exercised by a living membrane upon salts 

 in solution, or how they pass through it into the plant. 

 The observations of Graham (' Phil. Mag.' ser. IV. August 

 1850) show that matters capable of exerting a chemical 

 action upon animal membranes, such as carbonate of 

 potash and caustic potash, causing them to swell and 

 gradually decomposing them, facihtate the passage of 

 water to an extraordinary degree.* Graham remarks 

 that the processes of alteration, decomposition, and new 

 formation, wliicli are incessantly taking place in tlie 

 membranes and cells in all parts of the plant, and whicli 

 we liave no means of defining or measuring, must entirely 

 change the osmotic process : the permeation of mineral 



* The water in the tubes of his osmometer rose to 1G7 nullimeters, 

 when holding l/10»i. per cent, of carbonate of potash in sohition ; with 

 1 per cent, of that salt, it rose to 8G3 millimeters (38 inches, English). 

 In another experiment, the water holding 1 per cent, of sulphate of 

 potash in solution, rose to twelve millimeters; upon the addition of 

 I/IO per cent, of carbonate of potash to the solution, it rose to 254- 

 264 millimeters ; the .same potash solution by itself rose only to 92 

 millimeters. The notion of an osmotic equivalent is altogether inad- 

 missable, if the membrane is chemically altered. Graham's latest 

 investigations on the dialysis of crystalline and amorphous bodies are 

 extremely interesting, and promise to throw considerable light upon 

 the processes in the animal organism. 



