26 THEORIES 



something r , an occult quality in the atmosphere." (Med. 

 Cf-az., xvi. p. 515.) 



But thinking men can scarcely rest satisfied with nega- 

 tive conclusions, and therefore, new explanations of the 

 cause of fever, succeed to the uprooted theories of the age 

 gone by. 



The opinion which, next to that of "the malarial," 

 seems to be most successfully sustained, refers intermit- 

 tents to the obvious conditions of the air, altered by heat 

 and moisture ; hot days followed by. cool evenings, dry 

 days, by dewy nights. The strongest argument for this 

 conclusion, rests on the fact, that such diseases prevail at 

 the season of greatest contrast, both as to heat and hu- 

 midity, and in places where extensive wet grounds aid in 

 the production of the strong vicissitudes. 



As these phenomena are subject to observation, a close 

 examination may be made of the relative condition in 

 such respects, of the most healthy and the most deadly 

 localities. The result is not favorable to the theory based 

 on them, for many very salubrious places are remarkable 

 for the most striking manifestations of the supposed causes 

 of intermittents, while very sickly situations are not un- 

 frequently distinguished by the uniformity of the climate, 

 and the steadiness of the temperature and dew-point; nay, 

 two places, in all observable respects alike in elevation, 

 local relations, atmospheric phenomena and geological 

 structure, may differ totally in their degree of healthful- 

 ness. Even in the same place, the line of limitation of 

 disease-producing power, may be a common road, a nar- 

 row street, a stone wall, or a belt of woods; things which 

 could scarcely be supposed to affect, sensibly, the heat 

 and moisture, or their fluctuations. 



But the most fatal argument against this theory is the 



