FEVERS. 557 



" The effects of the air may be referred either to its sensible 

 qualities, viz. Heat and Cold, Moisture and Dry ness ; or to its 

 insensible qualities, viz. the various matters with which the air 

 may, as a menstruum, be impregnated. I refer to the air, ac- 

 cordingly, whatever is applied to our bodies in the way of vapour, 

 effluvia, &c. It may be expected, that I should have here 

 mentioned the essential properties of the air, as its gravity and 

 elasticity ; and these no doubt have their effects, but I cannot 

 perceive that there is any suspicion that fevers do arise from 

 any change in these properties. I go on therefore upon my 

 limited plan with respect to the influence of the sensible or 

 insensible qualities of the air. Physicians have been much di- 

 vided with respect to the question, how much ought to be attribut- 

 ed to the one, and how much to the other of these, and whether 

 they are to be considered as acting separately or in conjunction. 



" Hippocrates, our greater leader in this subject, in enumer- 

 ating the epidemics of the time in which he lived, has taken 

 such pains with respect to the sensible qualities, that he has 

 been understood by modern authors as if he had attributed all 

 the effects to them. In like manner, Wintringham, sen. has en- 

 deavoured to ascribe the epidemic diseases of Britain, for 

 twenty years, to the sensible qualities of the air. And Hux- 

 ham also has done much the same ; but his situation at Ply- 

 mouth led him to see that contagions had a great share, so that 

 he is not determined to conclude positively with respect to 

 either supposition. Neither was Hippocrates entirely of one 

 system. He speaks of the TO $>, or somewhat from above, 

 which may be understood as the insensible qualities of the air. 

 Sydenham again has, upon the other hand, taken up this view 

 only of ro $v. He supposes that epidemics do not depend 

 for their origin upon the sensible qualities of the air, but upon 

 certain miasmata, as he terms it. For my own part, I have 

 no doubt in declaring, that both Wintringham and Sydenham 

 are in extremes. I have already endeavoured to shew, that 

 miasmata and contagion are the chief causes of fevers, and so 

 far I support Sydenham ; but the sensible qualities of the air 

 also, may, in certain circumstances, produce Epidemic Pyrexiae, 

 and may modify fever, strictly so called, which properly arises 

 from the insensible qualities. . 



