HOSACK ON THE LAWS OP CONTAGION. 237 



tainted atmosphere, and thereby decomposing the morbid compound ? 

 According to Dr. Crawford, " the fluids which destroy the foetid odours 

 most speedily are those which are acknowledged to contain the greater 

 portion of oxygen, and it is, therefore, extremely probable that 

 this change depends on the union of the oxygen with animal hepatic 

 gas, or some one of its constituent parts." But the explanation which 

 has been offered by the late Dr. Garnett, of the manner in which the 

 oxygen thus employed combines with the hydrogen gas which holds 

 the morbid secretions in solution, appears to me the most satisfactory 

 explanation that has been given of those phenomena.* 



From these facts, I have been led to conclude, 



1st. That an impure atmosphere is indispensably necessary to mul- 

 tiply and extend the specilic poison constituting plague, dysentery, 

 typhus, and yellow fever. 



2dly. That the impurities of the atmosphere do not produce their 

 effects, in the manner suggested by Dr. Chisholm, by increasing the sus- 

 ceptibility of the system to be acted upon by the peculiar virus of those 

 diseases. 



3dly. That, instead of predisposing the body to be thus acted upon, 

 the reverse is the fact; that the predisposition of those who are most 

 exposed to such impure air is less, while those who reside in the pure air 

 of the country are most, liable to be infected when exposed to the con- 

 tagion. 



4thly. That the impurities of the atmosphere are fermentable mate- 

 rials, to be called into action by the specific ferment of those diseases, 

 aided by heat, moisture, and a calm state of the atmosphere, and that 



* Proceedings of the Board of Health of Manchester, p. 40 — i2.— Robertson's Treatise on 

 Medical Police, vol. 2. p. 127.— Robertson's Natural History of the Atmosphere, vol. 2. p. 352. 

 —See Note M. 



