THE EARTH. 12$ 



fered convulsions, and at last lay still, and being taken out, recovered 

 no motion at all, but was dead. A bird inclosed in artificial air, fron; 

 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 vacuo for several 

 hours : so that factitious air proved a greater enemy to animals thar 

 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, instantly destroy it. 

 We have already mentioned the vapours in the grotto Del Cane suf- 

 focating 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 wholesome soever in itself, becomes 

 infectious ; and that we owe the salubrity of the air to the variety of 

 its mixture. 



But there is no animal whose frame is more sensibly affected by the 

 changes of the air than man. It is true, he can endure a greater va- 

 riety of climates than the lower orders generally are able to do; but it 

 is rather by the means which he has discovered of obviating their ef- 

 fects, than by the apparent strength of his constitution. Most other 

 animals can bear cold or hunger better, endure greater fatigues in 

 proportion, and are satisfied with shorter repose. The variations of 

 the climate, therefore, would probably affect them less, if they had the 

 same means or skill in providing against the severities of the change. 

 However this be, the body of man is an instrument much more nice- 

 ly sensible of the variations of the air, than any of those which his 

 own art has produced ; for his frame alone seems to unite all their 

 properties, being invigorated by the weight of the air, relaxed by its 

 moisture, enfeebled by its heat, and stiffened by its frigidity. 



But it is chiefly by the predominance of some peculiar vapour, that 

 the air becomes unfit for human support. It is often found, by dread- 

 ful experience, to enter into the constitution, to mix with its juices, 

 and to putrefy the whole mass of blood. The nervous system is not 

 less affected by its operations ; palsies and vertigoes are caused by its 

 damps ; and a still more fatal train of distempers by its exhalations, 

 In order that the air should be wholesome, it is necessary, as we have 

 seen, that it should not be of one kind, but the compound of several 

 substances ; and the more various the composition, to all appearance 

 the more salubrious. A man, therefore, who continues in one place, 

 is not so likely to enjoy this wholesome variety, as he who changes 

 his situation ; and, if I may so express it, instead of waiting for a reno- 

 vation of air, walks forward to meet its arrival. This mere motion, 

 independent even of the benefits of exercise, becomes wholesome, by 

 thus supplying a great variety of that healthful fluid by which we are 

 sustained. 



A thousand accidents are found to increase these bodies of vapour, 



that make one place more or less wholesome than another. Heat may 



raise them in too great quantities ; and cold may stagnate them, 



Minerals may give off their effluvia in such proportion as to keep away 



VOL. i. I 



