GRAVITATION AND COHESION. 203 



tween its own particles, or portions, would all act in 

 favour of the descent and the convergence, whereas 

 they act in opposition to the ascent and the spread 

 Were that not the case, we should have water rising 

 in showers, just as often as it falls in showers, or, to 

 speak more correctly, there would neither be the 

 one nor the other ; because, wherever it happened 

 to be, the water would remain quite stationary. 



To understand well how nature works, it is abso- 

 lutely necessary to have clear and perfect views of 

 what may be called her elementary working, that 

 working in which there is no organization of parts, 

 and no individual substance which we can in any 

 way distinguish and observe. That, though it is not 

 the first we come to, inasmuch as it is not apparent 

 to the senses, is the true beginning of observation; 

 and unless we comprehend it, we lose the greater 

 part both of the pleasure and the profit of the obser- 

 vation of individual things. 



The cause of descent is gravitation ; the cause of 

 aggregation, or bringing together, or condensation 

 of any kind, is cohesion ; and the only force which 

 we know that can act in opposition to, or overcome, 

 either or both of these, is heat. Gravitation is, as it 

 were, the tie of all matter, without reference to any 

 thing in particular kinds of matter, but just their 

 quantities. Cohesion is the particular tie which 

 holds together the several kinds of matter, and it is 

 perhaps the ultimate foundation of all their differ- 

 ences. The motion of heat overcomes gravitation 

 only by loosing cohesion, by so dispersing the parts 

 of a substance as that they shall rise upward through 

 a substance, specifically lighter than that which their 

 form was before they were dispersed. Thus, when 

 heat acts so as to expand, and thereby to elevate, 

 it has always two resistances to contend with; 

 whereas, when heat is diminishing, and concentra- 

 tion and descent are taking place, these two act 

 jointly against the heat ; and both of them act with 



