558 HISTORY OF THERMOTICS. 



Sect. 2. Prelude to Daltons Doctrine of Evapo- 

 ration. 



VISIBLE clouds, smoke, distillation, gave the no- 

 tion of Vapour ; vapour was at first conceived 

 to be identical with air, as by Bacon 1 . It was 

 easily collected, that by heat, water might be con- 

 verted into vapour. It was thought that air 

 was thus produced, in the instrument called the 

 ceolipile, in which a powerful blast is caused by 

 a boiling fluid ; but Wolf showed that the fluid was 

 not converted into air, by using camphorated spirit 

 of wine, and condensing the vapour after it had 

 been formed. We need not enumerate the opinions 

 (if very vague hypotheses may be so termed,) of 

 Descartes, Dechales, Borelli 2 . The latter accounted 

 for the rising of vapour by supposing it a mixture 

 of fire and water ; and thus, fire being much lighter 

 than air, the mixture also was light. Boyle en- 

 deavoured to show that vapours do not permanently 

 float in vacuo. He compared the mixture of vapour 

 and air to that of salt and water. He found that 

 the pressure of the atmosphere affected the heat 

 of boiling water; a very important fact. Boyle 

 proved this by means of the air-pump ; and he and 

 his friends were much surprized to find that when 

 air was removed, water only just warm boiled vio- 



1 Bacon's Hist. Nat. Cent. i. p. 27. 



2 They may be seen in Fischer, Geschichte der Physik, vol. ii. 

 p. 175. 



