618 



LECTURE L. 



ON COHESION. 



Those properties of matter, which we have lately examined, if they are not 

 absolutely inseparable from its constitution, are, at least, always found attached 

 to such matter as we are able to submit to our experiments. There are, however, 

 many other general affections, to which all matter appears to be liable, 

 although none is perpetually subjected to them, and these are principally, if 

 not entirely, dependent on the force of cohesion. 



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 hghter than this, but they appear to be extremely porous; and perhaps 

 the solid substances of some of the celestial bodies may also be a Httle more 

 rare. It frequently happens, that the compression of an elastic fluid alone is 

 suflicient 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, pro- 

 vided 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, not- 

 withstanding 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 



