HOSACK ON THE LAWS OF CONTAGION. 213 



" May this agent reside in the atmosphere ? Does this peculiar con- 

 stitution of the air consist in a superabundance, or diminution, of the 

 ordinary proportion of oxygen in the atmosphere ? or in the combina- 

 tion of some peculiar gas or gases diffused in it ?" He suggests that a 

 series of eudiometrical and other observations, continued for several 

 years, might throw some light on this subject : " Time alone," he adds, 

 " may unfold this mystery."* But when we take into view the facts he 

 has already stated, relative to Rosetta, and are told by the same author, 

 that in Egypt the plague prevails when the Nile is low, and of course the 

 air loaded with the impurities thence arising ; that at Constantinople, 

 the cold weather, in winter, puts a period to its progress, and the still 

 more general observation, that the extremes both of heat and cold, are 

 unfavourable to the propagation of plague ; these facts, in connexion 

 with those already stated of this disease, as it has appeared at different 

 times, and in different parts of the world, are certainly calculated to 

 dissipate much of the mystery in which this subject has been enve- 

 loped. The remarks of Dr. M'Gregor, that the plague varies its type 

 according to the state of the air, and other circumstances, and that by 

 ventilation, fumigation, and attention to cleanliness, the progress of the 

 disease was arrested,f also serve to confirm the correctness of the view 

 which has been taken of this subject. Even the writings of Assa- 

 lini, who disbelieves the communication of this disease by contagion, 

 furnish additional support to the principle here contended for ; for he y 

 admits, that when persons are shut up and crowded together, in infected \ 

 places, the disease is readily contracted.:}; In another place, he observes, 

 " that if a person be exposed to breathe the infected air in the cham- 

 ber of a patient, or should he stay too long in the same atmosphere, he 



* Travels in Egypt, p. 533. f Med. Sketches, p. 111. $ Observations sur la Peste. 



