[ 2?0 ] 



vapov:r /';; Viicuo, or in rarificd air, than in air of greater denfity j 

 becaufe, as he well remarks, all external preffure refifts the ex- 

 panfion of v^'ater into vapour, Hygrom. 219. It is true that he 

 adds, in the fubfequent page, that, in a given time, water lofes 

 more of its weight by expofure to the open air than it does in vacuo ; 

 but to prove this fad he only quotes the experiment mentioned 

 in the 55th vol of the Phiiofophical Tranfadtions, p. 146*, v^here 

 indeed it is ftated, that a cup of water {landing under an exhaufted 

 receiver, lofl only 2,5 grains in 24 hours, whereas the fame quan- 

 tity of water expofed to the open air during that period of time, loft 

 35 grains. But it is plain that, even fuppofing the temperature and 

 furfaces of the water in both cups equal, the refiduary air under the 

 receiver (whofe capacity is not mentioned) muft have been fatura- 

 tcd long before that in the room could have been fo, and thus its 

 evaporation fpeedily checked. It is exprefsly faid that the air under 

 the receiver was only 43 times rarefied ; enough therefore re- 

 mained to be foon faturated: but if it had been ever fo perfedlly 

 exhaufted, the elafticity of the vapour already raifed would have 

 been fufficient to check farther evaporation. Befides, fince it is 

 acknowledged that water evaporates more eafily in vacuo than in 

 open air, it neceflarily follows that, if the temperature remained 

 the fame, and if the vapours had liberty to efcape, it would alfo 

 evaporate more copioufly. The quantities evaporated in fmall 

 portions of time are thofe that fhould be compared with each 



other i 



• By uiiftake he quotes the 59th vol. 



