672 HISTORY OF 



lay still, and being taken out, recovered no mo- 

 tion at all, but was dead. A bird enclosed in arti^ 

 ficial air, from raisins, died in a quarter of a 

 minute, and never stirred more. A snail was 

 put into the receiver, with air of paste ; in four 

 minutes it ceased to move, and was dead, although 

 it had survived in vacua for several hours: so 

 that factitious air proved a greater enemy to ani- 

 mals than even a vacuum itself. 



Air also may be impregnated with fumes that 

 are instantly fatal to animals. The fumes of hot 

 iron, copper, or any other heated metal, blown 

 into the place where an animal is confined, in- 

 stantly destroy it. We have already mentioned 

 the vapours in the grotto Del Cane suffocating a 

 dog. The ancients even supposed, that these 

 animals, as they always ran with their noses to 

 the ground, were the first that felt any infection. 

 In short, it should seem that the predominance 

 of any one vapour, from any body, how whole- 

 some soever in itself, becomes infectious; and 

 that we owe the salubrity of the air to the variety 

 of its mixture.* 



[* " The immense mass of permanently elastic fluid which surrounds the 

 globe we inhabit, must consist of a general assemblage of every kind of air 

 which can be formed by the various bodies that compose its surface. Most 

 of these, however, are absorbed by water; a number of them are decomposed 

 by combination with each other ; and some of them are seldom disengaged 

 in considerable quantities by the processes of nature. Hence it is that 

 the lower atmosphere consists chiefly of oxygen (vital air) and nitrogen 

 (foul air), together with moisture and the occasional vapours or exhalations 

 of bodies. The upper atmosphere seems to be composed of a large propor- 

 tion of hydrogen (inflammable air), a fluid of so much less specific gravity 

 than any other, that it must naturally ascend to the highest place, where* 

 being occasional^ set on fire by electricity, it appears to be the cause of the 

 aurora borealis and fire-balls." 



