58 THEORY OF FOOD-ABSORPTION. 
the micelle or groups of molecules, narrower apertures between the molecules 
or groups of atoms in each micelle, 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 affeeted 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 micellee 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 gases. 
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 micelle 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 
