REPORT OX THE LAWS OF CONTAGION. 77 



their source. As respects the emanations of the plague, this 

 has been attested by several writers. 1. Dr. RusseU, the author 

 of an excellent History of the Plague, preserved himself from 

 that disease, during a residence of several years at Aleppo, by 

 avoiding a nearer approach to the sick than four or five feet *. 

 Mr. Howard's experience satisfied him that, in a still atmo- 

 sphere, twelve feet was a perfectly safe distance f. Assalinitook 

 no other precaution, than to avoid inhaling the breath of persons 

 under that disease. 2. Smallpox infection was believed by Dr. 

 Haygarth, not only from his own experience but from a series 

 of experiments conducted by Dr. O'Ryan, of Lyons, not to ex- 

 tend beyond half a yard from the patient ; and that of typhus 

 to be at least as limited J. 3. Scarlatina, when introduced by 

 a new comer into a school, has generally been observed to spread 

 first to those associated in the same class, or otherwise, with the 

 infected person. On these facts is founded the salutary practice 

 of separating the sick from tlie healthy, on the first appearance 

 of a contagious malady; by which, in numberless instances, its 

 progress has been effectually stopped. It is a happy consequence, 

 also, of the limited extent of the infectious circle, that in a well 

 aired apartment, all those soothing and beneficial ministrations, 

 that do not require a very close approach to the sick, may be 

 performed with little if any danger to the attendants. 



XIX. It is impossible, however, to assign, to any species of 

 contagious emanation, distinct and constant boundaries. Even in 

 each particular instance, these limits are necessarily liable to fre- 

 quent variation. For, 1. The more abundant the production of 

 contagious effluvia, the wider, costeris paribus, will be the area 

 over which they will be diffused. 2. Imperfect ventilation ex- 

 tends the diameter of the infectious circle, and renders the poison 

 efficient at distances where, by due dilution with atmospheric 

 air, it would have been perfectly inert. Even the poisonous 

 gases prepared by chemical processes, it is well known, may, if 

 largely diluted with atmospheric air, be respired for a certain 

 time, without even the slightest injury. By availing ourselves 

 then of the law, which renders a certain state of concentration 

 essential to the activity of volatile contagions, it is easy to ob^ 

 tain complete exemption from their deleterious effects. Abun- 

 dant dilution, indeed, effected by well planned and assiduous 

 ventilation, is the most certain, if not the only, means of secu- 

 rity against contagious emanations, as they issue from the sick. 



XX. The i^voce^&fli spontaneous diffusion is too slow to ac- 



« Russell On the Plague, 4to, 1791, p. 99. 

 + On Lazarettos, p. 34 ; and Appendix to that work, p. 31. 

 X Inquiry, p. 97; and Sketch of a Plan to exterminate Smallpox, p. 237; 

 also Letter to Dr.Percival, p. 9. 



