PLANT NUTRITION. 11 
be the result of the displacement say of one molecule of 
water by one molecule of spirit, and so, throughout the 
whole quantity of liquid, there is displacement and re- 
placement until at length equilibrium is restored and a 
thorough diffusion results. This power of diffusion does 
not always exist. The molecules of water and of oil will 
not mix or diffuse freely through each other. Water 
containing carbonic acid gas will not mix, in this sense 
of the term, with water containing acetate of lead ; and 
when the attempt is made, chemical change is set up, 
‘the heretofore clear solutions—that containing the gas 
and that containing the lead.-become when combined 
opaque and milky, owing to a chemical change, resulting 
‘in the formation of a white insoluble lead carbonate: 
It may be a truism to say, that for the process of dif- 
fusion the liquids must be diffusible, but the fact’ must 
be carefully borne in mind in all questions relating to the 
‘feeding of plants. In the case of plants, the phenomenon 
of diffusion, or the gradual admixture of two liquids of 
different natures, is complicated by the presence of a 
membrane in the shape of the cell-wall. The water from 
the outside has to pass through the membrane to reach 
the protoplasm on the other side. Speaking broadly, 
there are no holes in the membrane through which the 
water can pass. Ingress is secured by that process of dif- 
fusion to which reference has just been made, and by 
virtue of which the molecules of the membrane and the 
molecules of the water shift and change places; the 
space that was occupied by a molecule of membrane is 
now occupied. by a molecule of water, and vice versa. 
The access, therefore, of water into the interior of a 
closed cell is the result of the process of diffusion. 
‘Where two liquids mix without any intervening mem- 
brane, the mixture is called diffusion simply ; where 
there is an in tervening membrane, the diffusion Panen 
is known as “‘ osmosis.” 
