58 THEORY OF FOOD-ABSORPTION. 



the micella? or groups of molecules, narrower apertures between the molecules 

 or groups of atoms in each micella?, and lastly the finest pores between the atoms 

 themselves in each molecule. These interspaces are liable to contraction and 

 expansion, for the union of the molecules is affected by two forces, one of which 

 manifests itself as a mutual attraction between atoms and atomic groups, whilst 

 the other tends to drive atoms and molecules asunder. Of these forces the former, 

 i.e. the attractive force existing in all material particles, is called chemical affinity 

 when it causes atoms of different kinds to unite to form a molecule; and it is called 

 cohesion when applied to the mutual attraction of similar molecules, and adhesion 

 where it holds together masses of molecular groups with their surfaces in contact. 

 The action of heat is opposed to this attractive force, which is only effective at 

 infinitesimal distances. Bodies are all caused to expand by heat, their atoms, mole- 

 cules, and micella? being forced apart. Heat is believed to be a vibratory motion 

 of these ultimate particles, and it is supposed that the greater the vibrations the 

 greater is the separation of atoms and atomic groups, the interspaces expanding 

 and the heated body increasing consequently in volume. As is well known, the 

 atoms and molecules may be forced so far apart by increase of temperature that 

 cohesion is entirely overcome, and solids are converted, first into liquids and at 

 last into eases. 



o 



The interspaces or passages between the molecules and molecular groups com- 

 posing a cell-membrane are penetrable by molecules of other substances, provided 

 always, firstly, that the admitted molecules are not larger than the passages; and 

 secondly, that there exists between the molecules of the cell-wall and those of the 

 penetrating body that sort of attractive force which has been designated chemical 

 affinity. Both premises are satisfied in the case of aqueous molecules, and experi- 

 ment proves that they are admitted into the inter-molecular spaces of a cell- 

 membrane with great ease and readiness. The cell-membrane saturates itself with 

 water, or, to use the technical phrase, it has the tendency and ability to "imbibe" 

 water. The force of attraction between molecules of a cell-membrane and water- 

 molecules is indeed so intense that the cohesion of the molecules in the membrane 

 is partially neutralized, and the imbibed water causes them to move apart. In 

 consequence of this, the cell-membrane swells up and its dimensions are increased. 



It is also supposed that the micella? of a cell-membrane attract and admit water- 

 molecules to such an extent as to surround themselves with watery envelopes. 

 Such a condition would no doubt be nothing but beneficial, promoting, as it would, 

 the interchange of materials through the cell-membrane, and the mixing of fluid 

 substances situated on either side of the porous membrane. At all events this 

 mixing process must ensue in the interspaces of the cell-membrane: and, in the 

 particular case out of which this discussion has arisen, viz. food-absorption, the 

 interacting substances are, on the one hand, the compounds in the soil outside 

 the cell-membrane, and, on the other, the organic compounds under the control 

 of the live protoplast within the cell-membrane. Both the outgoing and the in- 

 coming substances must be soluble in water, and must, therefore, have an attraction 



