268 Transialroii of Key's E$sai/s. 



which the weighing is performed. I proceed to deduce the 

 cause of this, which few persons have discovered. Whatever 

 gravitates in air (the same may be said of water) divides it, 

 pushes it aside, and makes it give place, in order to sink to the 

 bottom of it. This is called exercising its force and action 

 in air. Now it is a fact, that no agent acts in its like, all ac- 

 tion presupposing some difference. One hot body has no 

 action in another equally hot, but rather the two will embrace 

 and unite their actions, and by this union will no longer be two 

 agents but only one. But if a very hot body acts in that which 

 is less so, it is because in this case there is a dissimilitude, and in 

 some respects an opposition; the less hot claiming the title of cold, 

 with reference to the hotter. Thus air cannot act by its weight 

 in air equally heavy, the two airs rather unite, and make one 

 same weight. But whatever is heavier than air, by the dissimi- 

 litude and opposition, arising from this greater or less, will 

 act in it, dividing, pushing it aside, and making itself a way 

 through it to get to the bottom. But if air evince not its 

 weight in air itself, on account of the equality of their weights, 

 it follows a fortiori that it will not evince it in water, which is 

 heavier. For even if it be placed below, it will descend no 

 lower, the weight of the water above it only serving to compel 

 it to seek a higher place, not allowing it a station under 



itself. 



Essay IX. 



Air is rendered heavy by the mixture of some matter heavier than 

 itself. 



I PURPOSE to shew that it is the air, which mixing with the 

 calces of tin and lead, when they are calcined, increases their 

 weight ; which it would be impossible for me to do, without re- 

 movini' a no small difficulty that presents itself in this place. 

 For I may be asked, how can this that I assert be, since the 

 examination of this weight is effected by the balance, a7id in 

 air, where air can have no weight, according to the doctrine of 

 the preceding essay ? To clear up this doubt, I say, that portions 

 of the air may be changed and increased in weight, so that 

 these portions, so altered and made heavier, being weighed in 

 pure air, will afford evidence of their weight. But what is this 



