GENERAL LAWS OF EPIDEMICS. 823 



appear in many parts of the world, at intervals of 

 seventeen, thirty, or more years. We should then 

 know why epidemics prevail over continents, and 

 sometimes a whole hemisphere, for one, two, or 

 more seasons, and then disappear, or assume a 

 different type, why the black death differed in 

 some respects from the sweating sickness, and the 

 blue cholera from both, in other respects. What 

 medical authors term an epidemic constitution of 

 the atmosphere, is a mere cloak for ignorance of 

 the various physical changes that mark the 

 course of nature. To refer it to the influence of 

 comets, telluric emanations, and other occult 

 causes, is no better than the idle dreams of astro- 

 logy. 



We are told by medical authors, that fever arid 

 many other forms of disease, are generated by 

 malaria. But they have never yet informed us 

 what malaria is, nor how it produces intermittent 

 fever in one case, remittent fever in another case, 

 typhus fever, yellow fever, plague, cholera, dy- 

 sentery, diarrhea, &c. in other cases. This much, 

 however, is certain, that it proceeds from the de- 

 composition of dead organic matter, the principal 

 results of which are carbonic acid and water, with 

 very small proportions of carburetted hydrogen, 

 sulphuretted hydrogen, and phosphuretted hy- 

 drogen. But it is still undecided whether malaria 

 depends on a greater abundance than usual of 

 carbonic acid, or of some other gaseous emanation. 



