CAtJSES OF THE MOTION OF JUICES. 403 



osmose. Graham long ago observed the decomposition 

 of alum (sulphate of aluminium and potassium) by mere 

 diffusion ; its potassium sulphate having a higher diffu- 

 sive rate than its aluminium sulphate. In the same 

 manner acid potassium sulphate, put in contact with 

 water, separates into neutral potassium sulphate and 

 free sulphuric acid.* 



We have seen (pp. 170-1) that the plant, when veg- 

 etating in solutions of salts, is able to decompose them. 

 It separates the components of potassium nitrate appro- 

 priating the acid and leaving the base to accumulate in 

 the liquid. It resolves chloride of ammonium, taking 

 up ammonia and rejecting the hydrochloric acid. The 

 action in these cases we cannot definitely explain, but 

 our analogies leave no doubt as to the general nature of 

 the agencies that cooperate to such results. 



The albuminoids in their usual form are colloid 

 bodies, and very slow of diffusion through liquids. 

 They pass a collodion membrane somewhat (Schu- 

 macher), but can scarcely penetrate parchment-paper 

 (Graham). In the plant they are found chiefly in the 

 sieve-cells and adjoining parts of the cambium. Since 

 for their production they must ordinarily require the 

 concourse of a carbhydrate and a nitrate, they are not 

 unlikely generated in the cambium itself, for here the 

 descending carbhydrates from the foliage come in con- 

 tact with the nitrates as they rise from the soil. On the 

 other hand, the albuminoids become more diffusible in 

 some of their combinations. Schumacher asserts that 

 carbonates and phosphates of the alkalies considerably 

 increase the osmose of albumin through collodion mem- 

 branes (Physik der Pflanzen, p. 128). It is probable that 

 those combinations or modifications of the albuminoids 



*The decomposition of these salts is begun by the water In which 

 they are dissolved, and is carried on by osmose, because the latter 

 secures separation of the rein-tins substances. 



