How Plants Draw in Various Materials 175 



branes, water, and dissolved substances are all of them composed, 

 according to the teaching of physics, of ultimate excessively 

 small units, called molecules. In the solid or liquid state, the 

 molecules are held together by a force of mutual attraction, 

 called cohesion, analogous to the force which holds an armature 

 to a magnet. But when heat in sufficient amount is supplied, 

 there comes a point at which the cohesion of the molecules is 

 suddenly overcome and replaced by an opposite tendency to 

 spread or diffuse just as far apart as they can; and this is what 

 constitutes a gas. The power that actuates the diffusion is heat, 

 which, catching the tiny molecules in the swirl of the violently- 

 vibratory ethereal waves of which it consists, imparts to them 

 its own vigorous motion, whereby they are set swiftly darting 

 and dancing hither and yon, bounding and rebounding energet- 

 ically against one another, with a result that they work steadily 

 outward, very much as a cargo of corks would be spread from a 

 foundered vessel on the waves of a tempestuous sea. Familiar 

 examples of this diffusion of gases are many, for instance, the 

 spread and ultimate disappearance of odors, and the penetration 

 of cigar smoke though the house; but all gases diffuse in this 

 manner. And here comes a curious and consequential fact about 

 diffusion, namely, that it occurs not only in gases, but also in 

 anything, whether solid, liquid or gaseous, when dissolved in a 

 liquid. Examples thereof are abundant, the gradual spread of 

 a bit of solid dye when dropped into water: the spread of sugar 

 through coffee or tea without stirring if only tune be allowed: 

 the spread of fertilizers evenly through soil though added in large 

 lumps on the surface. By diffusion, also, the molasses reaches 

 the water outside of the tube of our osmoscope. Such diffusion 

 occurs, as it seems, because an adhesive attraction existing be- 

 tween the molecules of the substance and those of the dissolving 

 liquid separates the molecules of the substance from one another, 

 and thus brings them into a condition such that heat can exert 

 upon them the same action as it does upon the separated mole- 



