GRAVITATION AND COHESION. 209 



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 observation of individual things. 



The cause of descent is gravitation ; the cause 

 of aggregation, or bringing together, or condensa- 

 tion 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. Gravi- 

 tation is as it were the tie of all matter, without 

 reference to any thing* in particular kinds of mat- 

 ter, 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 cohe- 

 sion, 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, 

 whenheat is diminishing, and concentration and de- 

 scent are taking place, these two act jointly against 

 the heat ; and both of them act with vigour, en- 

 creasing inversely as the squares of the distances 

 from their centres of action. 



Even admitting that moisture floats in the 

 atmosphere to the highest elevation at which that 

 is estimated to have sensible weight, which is 

 about fifty miles above the mean surface, that is 

 only one-eightieth part of the distance of the mean 

 surface from the centre, so that, from mere gravi- 

 T 3 



