THEIR MODE OF ACTION. * 407 



of solid matters from the blood, by which the action 

 of the air upon the latter is altered or prevented. 

 -"When gaseous and decomposing substances, or 

 those which exercise a chemical action, such as sul- 

 phuretted hydrogen and carbonic acid, obtain access 

 to the lungs, they meet with less resistance in this 

 organ than 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. Sulphu- 

 retted hydrogen produces immediate decomposition 

 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 decomposition, 

 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 ani- 

 mal and vegetable substances, or if it acts by its 

 chemical properties, (not by the state in w^hich 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 with- 

 out 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 be- 

 come covered with water containing a certain quan- 

 tity of this matter in solution. This water soon 

 becomes turbid, and in common language putrefies. 



