406 POISONS, CONTAGIONS, MIASMS. 



state of decomposition to a matter in the blood, and 

 from a second matter is itself regenerated, but by a 

 totally different mode of decomposition; the product 

 possesses the mild form, and all the properties of 

 the lymph of cow-pox. 



The susceptibility of infection by the virus of 

 human smallpox must cease after vaccination, for 

 the substance to the presence of which this suscep- 

 tibility is owing has been removed from the body by 

 a peculiar process of decomposition artificially ex- 

 cited. But this substance may be again generated 

 in the same individual, so that he may again become 

 liable to contagion, and a second or a third vaccina- 

 tion will again remove the peculiar substance from 

 the system. 



Chemical actions are propagated in no organs so 

 easily as in the lungs, and it is well known that dis- 

 eases of the lungs are above all others frequent and 

 dangerous. 



If it is assumed, that chemical action and the vital 

 principle mutually balance each other in the blood, it 

 must further be supposed that the chemical powers 

 will have a certain degree of preponderance in the 

 lungs, where the air and blood are in immediate 

 contact; for these organs are fitted by nature to 

 favor chemical action; they offer no resistance to the 

 changes experienced by the venous blood. 



The contact of air with venous blood is limited to 

 a very short period of time by the motion of the 

 heart, and any change beyond a determinate point 

 is, in a certain degree, prevented by the rapid re- 

 moval of the blood which has become arterialized. 

 Any disturbance in the functions of the heart, and 

 any chemical action from without, even though weak, 

 occasions a change in the process of respiration. 

 Solid substances, also, such as dust from vegetable, 

 animal, or inorganic bodies, act in the same way as 

 they do in a saturated solution of a salt in the act 

 of crystallization, that is, they occasion a deposition 



