Chap. 9-] Vague Notions of the Ancients. 417 



water is put into a vefTel and heated, the ftratum of 

 air which adheres to the fides of the vefTel, and which 

 occupies a fituation between the water and the fides, 

 foon becomes perceptible there in the form of bubbles, 

 in confequence of the rarefaction which is caufed by 

 the heat. It becomes .fenfible in the fame manner in 

 a vacuum, in confequence of the dilation occalioned by 

 the preiTure being removed *. 



The ancients knew air to be a fluid, but their im- 

 perfect knowledge of thofe fubftances in general, ap- 

 pears to have difabled them from ufmg thofe means 

 which the moderns have employed for drawing off 

 and expelling this fluid from a certain fpace. They 

 were, indeed,, utterly unacquainted with the fact, that 

 air is a ponderous fluid. They admitted that there 

 were two kinds of bodies in nature; heavy bodies, 

 fiich as {tones, metals, and in general all bodies which, 

 being left to themfelves, had a propensity to defcend ; 

 and light bodies, fuch as air, flame, vapours, &c. be- 

 caufc thefe bodies appeared to them to afcend fponta- 

 neoufly into the upper regions of the atmofphere. 

 They fuppofed, therefore, agreeably to this fentiment, 

 that air was endued with abfolute levity; and that all 

 the effects which the moderns attribute to the principle 

 of gravitation, were to be afcribed to the horror which 

 nature had, according to them, for a 'vacuum. It was, 

 therefore, a long prevailing opinion, that air was defti- 

 t'.ite of weight : and it is not above a hundred and 

 fifcy years fince phjlofophers have been convinced of 

 this error. The engineers of the Count de Medici, 

 Great Duke of Florence, having received orders to 

 raife fome water fifty or fixty feet by means of a com- 

 mon -pump, perceived, when they made the attempt, 



* Brifibn, Tom. ii, p. 93. 

 VOL. I. E e that 



