HOSACK ON THE LAWS OF CONTAGION. 233 



is also proverbially true. It is also to be observed, as universally ad- 

 mitted, that the same disease has uniformly been extinguished by the 

 approach of frost, which destroys such fermentative process. 



Another argument in favour of this explanation is derived from the 

 fact that this disease has, in several instances, been introduced into our 

 cities, without extending beyond the individuals who have introduced it; 

 manifestly owing to the active exertions of a vigilant police, at the same 

 time that every attention was paid in preserving cleanliness about the per- 

 sons of the sick. This was remarkably the case in the year 1804, when the 

 yellow fever was introduced at the Wallabout, on Long Island, and in 

 1809, when the same disease prevailed at Brooklyn. In each of those 

 years the fever was introduced into this city by persons who had received 

 the infection on Long Island, but, owing to the circumstances just men- 

 tioned, it was not communicated to others; while the same disorder, 

 owing to local circumstances, spread in the vicinity of those places on 

 Long Island where it had first appeared.* 



During the year 1811, the yellow fever was also introduced into 

 the city of Amboy, New-Jersey, from the Havanna, but did not spread 

 beyond those persons who were first attacked in consequence of their 

 immediate exposure to the air of the infected vessel. The local cir- 

 cumstances of Amboy, its elevated situation, its dry and sandy soil, 

 its wide streets and spacious houses, their distance from each other, and 

 the remarkable cleanliness of the town, most satisfactorily account for 

 the sudden extinction of the disease, while the evidence of its impor- 

 tation must be admitted to be conclusive.! 



* American Med. and Phil. Reg. vol. 2. p. 95, &c. f See American Med. and Phil. Reg. 



toI. 3. ; also Edinburgh Med. and Surg. Journal, and the Med. and Phys. Journal of London. 

 — See Note I. 



32 



