H.OSACK ON THE LAWS OF CONTAGION. 203 



objecting to the third. After enumerating his several objections, he 

 requests me to reconsider my third division, which appears to him to 

 be the only objectionable one. This I have done, and now submit to 

 this society the result of a further examination of this subject, and a 

 detail of the facts by which I have been led to my conclusion relating 

 to the laws of communication, which I have more particularly assigned 

 to the febrile diseases enumerated in the third class. 



In my first communication, I acknowledge I have stated my observa- 

 tions without so full a detail of the facts themselves whence my con- 

 clusions were deduced, as perhaps ought to have been exhibited. To 

 the European reader, unacquainted with the peculiarities of yellow fever, 

 more especially as it has appeared in the cities of the United States, nry 

 first statement may perhaps appear defective in that evidence which is 

 so justly exacted upon subjects of this nature. This evidence I shall now 

 endeavour to supply, and thereby to confirm the correctness of the 

 classification which has been proposed. Waiving for the present all in- 

 quiry relative to the nature or properties of the contagious principle se- 

 creted by the diseased body, or the chemical qualities of the atmosphere 

 deemed necessary for its propagation, or the manner in which the con- 

 tagion diffuses itself, I proceed to observe, that the history of each dis- 

 ease enumerated in the third class, viz. plague, dysentery, typhus, in all 

 its forms, and yellow fever, furnishes evidence of the correctness of the 

 remark, that they are governed by a law peculiar to themselves, that 

 they are contagious or communicable in afoul atmosphere, but that they 

 are never or very rarely so in a pure air, where the sick enjoy the bene- 

 fits of cleanliness and ventilation. 



The same evidence, I trust, will demonstrate another truth, that 

 these diseases are, in no instances, epidemic, as they have been impro- 

 perly denominated by most practical writers, but that their sphere of 

 operation is, with very few exceptions, confined within the limits to which 



