474 LECTURE L. 



In order that any two particles of matter may cohere, it is necessary 

 that they be within a very small distance of each other, and the density 

 of any substance, composed of cohesive particles, must probably always 

 be more than half as great as that of water. There are indeed some solids 

 apparently a little lighter than this, but they appear to be extremely 

 porous : and perhaps the solid substances of some of the celestial bodies 

 may also be a little more rare. It frequently happens, that the compres- 

 sion of an elastic fluid alone is sufficient to cause the force of cohesion to 

 take place between its particles ; thus, if common steam be exposed, in a 

 close vessel, to a pressure greater than that of the atmosphere, it will be 

 wholly condensed into water, provided that no elevation of temperature be 

 allowed : and the same has been experimentally shown of many other 

 aeriform fluids, which may be reduced to liquids by pressure ; but others 

 of these fluids retain their elasticity, notwithstanding any force which 

 human art can apply to them. 



It is probable that as soon as the particles of any elastic fluid are brought 

 within the reach of the force of cohesion, it commences at once in its full 

 extent, so as to cause them to rush together, until it is balanced by that of 

 repulsion, which continually increases as the particles approach nearer to 

 each other ; they must then remain, perhaps after some vibrations, in a 

 state of equilibrium ; and if any cause should tend to separate them, or to 

 bring them nearer together, they would resist it, in either case, with a force 

 proportional to the degree of extension or compression. The distance at 

 which the force of cohesion commences, is not the same for all kinds of 

 matter, nor even for the same substance at different temperatures ; it is 

 smaller for vapours of all kinds, in proportion as their temperature is 

 higher, the cohesion itself being also smaller. If the experiments on the 

 density of steam have been correct, it follows that the force of repulsion 

 must increase more rapidly than the distances diminish, for the elasticity 

 of water is nearly ten times as great as that which would be inferred from 

 the compression of steam into a substance of equal density : this suppo- 

 sition agrees also with the experiments on the mean density of the earth, 

 which is probably not so great as it would be if the force of repulsion 

 increased in the simple ratio of the density. The law of repulsion appears 

 also to be in some degree modified by the effect of heat, which increases its 

 force at greater distances more considerably than at smaller. It appears 

 indeed, from the diminution of the elasticity of a spring by heating it, that 

 the repulsive force of the particles of bodies at very small distances is even 

 diminished by heat, unless the force be again supposed to decrease much 

 more rapidly than the distance diminishes : thus the diminution of the 

 elasticity of iron by heat is about thirty times as great as the increase of 

 the distance of its particles ; so that the original repulsive force must pro- 

 bably be somewhat diminished, although less than the cohesive force. At 

 greater distances, however, the force of repulsion is certainly increased ; 

 for the elasticity of vapours and gases of all kinds is evidently greater as 

 the temperature is higher. (Plate XXXIX. Fig. 530.) 



The cohesion of two or more particles of matter to each other does not 

 interfere with their power of repelling other particles situated in a different 



