THE EARTH. 217 



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 lor 

 a renovation of air, walks forward to meet its arrival. This 

 mere motion, independent even of the benefits of exercise, be- 

 comes wholesome, by thus applying 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 ano- 

 ther. Heat may raise them in too great quantities ; and cold 

 may stagnate them. IMinerals may give off their effluvia in such 

 proportion as to keep away all other kind of air ; vegetables may 

 render the air unwholesome by their supply; and animal putre- 

 faction 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 •, 

 b\it any one of them, for a long time singly predominant, be- 

 comes 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 considei 

 forty as a very advanced time of life, and generally die of old 

 age at fifty. At Carthagena,' in America, where the heat of 

 the hottest day ever known in Europe is continual, where, dur- 

 ing their winter season, these dreadful heats are united with a 

 continual succession of thunder, rain, and tempests, arising from 

 their intensentss, the wan and livid complexions of the inhabi- 

 tants might make strangers suspect that they were just recover- 

 ed from some dreadful distemper ; the actions of the natives are 

 conformable to their colour ; in all their motions there is some- 

 what relaxed and languid ; the heat of the climate even affects 

 their speech, which is soft and slow, and their words generallj 

 broken. Travellers from Europe retain their strength and ruddy 

 colour in that climate, possibly for three or four months ; but 

 afterwards suffer such decays in both, that they are no longer to 

 be distinguished from the inhabitants by their complexion. 



1 Ulloa, voJ. i. p. 42. 



