74 HISTORY OF 



ed: fey 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 renovation of air, walks forward to 

 meet its arrival. This mere motion, independent 

 even of the benefits of exercise, becomes whole- 

 some, 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 stag- 

 nate them. Minerals may give off their effluvia 

 in such proportion as to keep away all other kind 

 of air ; vegetables may render the air unwhole- 

 some by their supply ; and animal putrefaction 

 seems to furnish a quantity of vapour, at least as 

 noxious as any of the former. All these united, 

 generally make up the mass of respiration, and 

 Are, when mixed together, harmless; but any 

 one of them, for a long time singly predominant, 

 becomes at length fatal. 



The effects of heat in producing a noxious 

 quality in the air, are well known. Those torrid 

 regions under the Line are always unwholesome. 

 At Senegal, I am told, the natives consider forty 



