202 HOSACK ON THE LAWS OF CONTAGION. 



air, though more readily by means of the latter than the former, and 

 with which persons are rarely infected more than once in their lives. 



Under the third class are enumerated those diseases which are only, 

 in general, communicable, or contagious through the medium of an impure 

 atmosphere ; the air being rendered thus impure by the decomposition 

 of animal and vegetable substances, as in low, marshy countries ; or by 

 concentrated human effluvia, as in camps, jails, hospitals, or on ship- 

 board ; but the same diseases I alleged, in a pure air, in large and well 

 Ventilated apartments, when the dress of the patient is frequently 

 changed, all excrementitious discharges constantly removed, and at- 

 tention paid to cleanliness in general, are not usually contagious, or, 

 under such circumstances, are very rarely communicated from one per- 

 son to another. 



In this class I included the plague, dysentery, typhus fever, in its various 

 forms, of jail, ship, hospital, or lake fever, and the yellow fever. 



I also remarked, that these diseases, like many of the first class, may 

 be repeatedly contracted ; but that they are communicable, or other- 

 wise, according to the condition of the air in which they occur, or into 

 which they may be introduced : it was further observed, that the at- 

 mosphere thus impregnated by the peculiar virus emanating from the 

 diseased body, becomes assimilated to the poison, or ferment introduced, 

 and thereby is rendered capable of reproducing in others the same 

 specific disease, whether it be the plague, dysentery, typhus, or yellow 

 fever. Such are the outlines of my first communication to Dr. Chis- 

 holm.* 



In 1309 Dr. Chisholm did me the honour to reply! to my observa- 

 tions, expressing his entire approbation of the two first classes, but 



* See Note A. 



' See American Medical and Philosophical Register, vol. 2. p. 121. See NotelL 



