NUTRITION: THE WORK AND THE MATERIALS. 5 



Ingress and Movements of Water ; Diffusion, Osmosis. 



Our first inquiry, then, must be to ascertain how the 

 water, whose- presence in sufficient quantity we have as- 

 sumed, gets from without through the cell-membrane into 

 the protoplasm how, in fact, the first stage in indepen- 

 dent nutrition is accomplished. When one liquid, say 

 spirit, is poured into another, say water, the two gradually 

 mix. If we suppose these liquids to consist of a number 

 of molecules,* then, mixture may be taken to 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 replacement till 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 be- 

 come when combined opaque and milky, owing to a chemical 

 change, resulting in the formation of a white insoluble lead 

 carbonate. 



* It may be well, once for all, to explain the sense in which the term 

 "molecule " is here used. It is now generally assumed by physicists that 

 every substance in nature is made up of excessively minute particles called 

 atoms, which are indestructible. An atom cannot exist by itself, but in 

 association with others. Such a group of atoms is called a ' ' molecule. " 

 A molecule, therefore, is the smallest group of atoms capable of existing 

 separately and independently. These molecules may be of different sizes 

 in different cases, and they are believed to be so arranged as just not to 

 touch, but to leave spaces between them ; smaller in the case of a hard 

 solid, wider in that of a liquid, still wider in that of a gas. The extent, 

 moreover, of these interspaces may be increased or diminished by varying 

 degrees of heaL 



