THEIR MODE OF ACTION. 375 



organ tnan in any other. The chemical process of 

 slow combustion in the lungs is accelerated by all 

 substances in a state of decay or putrefaction, by 

 ammonia and alkalies ; but it is retarded by empy- 

 reumatic substances, volatile oils, and acids. 

 Sulphuretted hydrogen produces immediate decom- 

 position of the blood, and sulphurous acid combines 

 with the substance of the tissues, the cells, and 

 membranes. 



When the process of respiration is modified by 

 contact with a matter in the progress of decay, 

 when this matter communicates the state of decom- 

 position, of which it is the subject, to the blood, 

 disease is produced. 



If the matter undergoing decomposition is the 

 product of a disease, it is called contagion ; but if it 

 is a product of the decay or putrefaction of animal 

 and vegetable substances, or if it acts by its chemi- 

 cal properties, (not by the state in which it is,) and 

 therefore enters into combination with parts of the 

 body, or causes their decomposition, it is termed 

 miasm. 



Gaseous contagious matter is a miasm emitted from 

 blood, and capable of generating itself again in blood. 



But miasm properly so called, causes disease 

 without being itself reproduced. 



All the observations hitherto made upon gaseous 

 contagious matters prove, that they also are sub- 

 stances in a state of decomposition. When vessels 

 filled with ice are placed in air impregnated with 

 gaseous contagious matter, their outer surfaces 



