FEVERS. 545 



when every body is exposed, a certain, and sometimes a small 

 number only are seized, and some always escape. I shall offer 

 a few remarks upon this curious and important matter ; and I 

 observe, that the action of the effluvia will be according to their de- 

 gree of power, or that fevers will arise more or less readily as the 

 miasmata or contagions are more or less powerful. Now it may 

 be presumed that as these effluvia are the product of a certain 

 fermentation, this fermentation may be, upon different occasions, 

 in different degrees of activity, which will be one source of the 

 different matter produced ; or, the same matter may be accu- 

 mulated in different quantities, and possess a different degree of 

 concentration in a given space ; but the question still remains, 

 what is it that increases the activity and quantity of this matter ? 

 To this I answer directly, that heat is to be considered as the 

 chief instrument in giving activity to fermentation, and hence 

 may be the chief means of supporting the action of the efflu- 

 via, by increasing not only their activity but their quantity. 

 So, according to the heat of the season, the quantity and qua- 

 lity of the effluvia will be in proportion ; and the observations 

 which have been made will be found to be agreeable to this. 



" Fevers arising from miasmata may indeed appear in 

 northern climates, but their operation is more considerable in 

 the warm climates ; heat alone, however, is not sufficient ; in 

 dry bodies no fermentation takes place ; moisture is a necessary 

 circumstance, and it is only in humid grounds that noxious 

 miasmata arise. But it is not yet determined whether every 

 part of the earth's surface, properly acted upon, be a proper sub- 

 stance for producing this miasma, or if it must be impregnated 

 with a particular putrescent matter in order to produce it. There 

 are indeed cases of inundations which leave putrefying animal 

 matter exposed to the air ; but inundations from recent rains 

 also, which do not afford such a matter, produce noxious effluvia 

 in consequence of a certain degree of heat. And we have many 

 instances of earth being merely kept moist by the neighbour- 

 hood of woods, which upon occasion prove noxious. All earth 

 in a humid state accordingly contains a sufficient quantity of 

 putrescent matter ; or rather there is no part of the earth, from 

 which, after it has been exposed to a certain degree of moisture 



