CHAP. 5. ON VITAL FUNCTIONS. 185 



and being further varied by the particular state 

 of constitution, and other circumstances of the 

 patient.* 



Even contagious diseases break out at very 

 uncertain periods, and often without any obvious 

 cause, though they are afterwards evidently 

 propagated by infection. Parts of Turkey are 

 said to be visited by the plague every five or 

 six years, while the same disorder appears more 

 rarely in other places. The small pox rages for 

 a time throughout whole tracts of country ; at 

 others, there is scarcely a case to be met with :. 

 the same may be observed of scarlitina and 

 measles. I cannot persuade myself that this is 

 merely the effect of accidental introduction. 

 Is it possible there may be some quality in the 

 air, at particular times, whereby it is fitter for 

 the conveyance of infectious matter ? Or, can 

 we suppose the effect of a peculiar state of 

 atmosphere to be that of rendering the body 

 more susceptible of infection than ordinary. 



In artificial society there are so many causes 



* Particular classes of diseases also belong to particular 

 parts of the world. A correct catalogue of liabilities to,, and 

 exemptions from, diseases in particular places, would form 

 a useful appendix to Nautical Almanacks and Ephemerides 

 intended for foreign use. 



