223 HOSACK ON THE LAWS OF CONTAGION. 



the poison of yellow fever, with the peculiar virus itself, by which that 

 disease has been introduced into the various cities of the United States. 



The same local circumstances, I believe, will go far in accounting 

 for the " pestilential state of the air," the " secret constitution of 

 atmosphere," so often recorded by writers on epidemics; at the same 

 time that they teach us that the diseases now under consideration are 

 only epidemic in as far as the vitiated state of the air is itself epidemic. 



I, however, wish it to be understood, that I do not exclude the influ- 

 ence of bodily predisposition, the passions of the mind, and many other 

 circumstances, in aiding the propagation of pestilential diseases. 



Having, as I trust, shown, by the facts that have been adduced, that 

 the plague, dysentery, typhus, and yellow fever, constituting the third 

 class of contagious diseases, require an impure state of the air to diffuse 

 and multiply them, the question next presents itself, in what manner 

 does such impure air operate in spreading those diseases? Upon this 

 part of the subject I have the misfortune to differ from Dr. Chisholm 

 no less than I do as to the necessity of such an atmosphere to propa- 

 gate the peculiar poison of each of those diseases. Dr. Chisholm ob- 

 serves, that if the proposition had been advanced, " that those diseases, 

 particularly the pestilential yellow fever, are rendered more violent in 

 their action under the circumstances stated, of an impure atmosphere, 

 that no possible objection could be made to it, inasmuch as it is sup- 

 ported and proved by all experience ;" and he proceeds to express the 

 opinion that such an atmosphere may have an effect " by rendering the 

 system of the healthy person, who/ receives the poison from the sick, 

 more susceptible at the moment of its introduction, of its peculiar ac- 

 tion;" but that this multiplying power does not proceed from any actios 



