PROPAGATION OF EPIDEMICS. 237 



here to refer, for believing that these disorders 

 of large masses of people are produced by some 

 peculiar organic poison, not gaseous, nor vapor- 

 ous. It is at least very certain that such organic 

 particles as are detrimental to the health, float 

 in the atmosphere of every great city, and may 

 often be perceived by the senses in the offen- 

 sive air of the habitations of its poor and dirty 

 inhabitants. Such particles cannot exist in the 

 air without \mdergoing chemical change, and it 

 is possible that by their existence in this state 

 of change, they may set in motion a series of 

 events which terminates in the appearance of 

 the disease we are alluding to. A simple 

 experiment will prove the truth of the assertion 

 that an atmosphere of organic matters is un- 

 doubtedly mixed with our air. It is a property 

 of strong sulphuric acid to char or blacken most 

 substances of this kind ; now, if a saucer, partly 

 filled with this acid, is exposed for a little period 

 to the air, its colour will alter, and ultimately 

 deepen almost to black, in consequence of a 

 large portion of organic matter having fallen into' 

 it from the air, and undergone the charring pro- 

 cess; and this will take place even in the open 

 air of the country. 



As we look down upon yon distant village, 

 it is not necessary to call in the aid of che- 

 mistry to inform us that even its compara- 



