92 FOURTH REPORT — 1834. 



6. All the predisposing causes enumerated as promoting the 

 spread of contagious diseases, conti-ibute also to thatof epidemics. 

 The latter, also, are propagated by some causes of general ope- 

 ration, such as a scanty harvest, or produce of bad quality ; a 

 severe winter ; a scarcity of fuel ; an unusually crowded popu- 

 lation ; and, on some occasions, harassing and destructive wars. 

 In some instances, the path has been prepared for one epidemic 

 hy the previous ravages of another : in other examples, the new 

 epidemic has acquired an ascendency over existing ones, and 

 has either modified or entirely extinguished them. In 1666 the 

 plague imparted much of its own form to a low petechial fe- 

 ver prevailing in London, but minor diseases for a while disap- 

 peared. Even the smallpox was superseded by the more power- 

 ful malady. 



7. In what the influence of atmospheric changes in causing or 

 diffusing epidemics consists, it is impossible, in the present state 

 of our knowledge, to explain. The most diligent observation has 

 not connected the prevalence of those maladies with any ascer- 

 tained condition, either physical or chemical, of the general atmo- 

 sphere. With respect to oxygen and nitrogen gases, which consti- 

 tute, at a mean of the barometer and thermometer, 98-J- in 100 of 

 its volume, an almost perfect uniformity is known to exist. In its 

 carbonic acid no variation has been discovered by experiment, 

 that can be supposed to affect the animal oeconomy. The varieties 

 of proportion in its aqueous vapour are, however, much greater ; 

 and when accompanied, as they often are, by sudden changes of 

 temperature, and by disturbances of the electrical equilibrium, 

 may interrupt the due performance of the bodily functions. But 

 other causes are necessary to account for those epidemics (cho- 

 lera, for instance,) which defy the influence of climate, seasons, 

 and of all changes that are objects of meteorological research. It 

 has been suggested that an ' epidemic constitution ' of the atmo- 

 sphere may depend on the presence of some substance alien to 

 its ordinary elements. No fact, however, confirms this suppo- 

 sition, if we except an observation of Dr. Prout, that at a period 

 coinciding with the appearance of cholera in London, the weight 

 of a given volume of air, making due corrections for differences 

 of pressure and temperature, seemed to rise to a small but sen- 

 sible amount above the visual standard, and continued above it 

 during six weeks*. This observation requires, however, to be 

 frequently and carefully repeated, and extended to other epide- 

 mics, as opportunities occur, before any sound conclusion can 

 be founded upon it. 



8. Epidemics have been contrasted with contagious diseases, 



* Bridgewater Treatise, p. 350. 



