300 On Malaria. 



without number, prove that space, bulk, or extent is not essen- 

 tial ; but that the smallest spot of the same character, or of the 

 proper character, is sufficient to produce disease, though the 

 extent of its influence must be expected to vary in consequence : 

 and even without these facts, this would be judged true from 

 the following argument. 



It is ascertained that a marshy spot can influence to disease 

 a place situated at a considerable distance from it ; and this 

 extent, in Italy, has been ascertained to amount to at least 

 three miles. Now it is plain, that whatever the original body 

 of malaria was, it must have been diluted by the atmosphere 

 during such a course, to a very great degree, or that the quan- 

 tity reaching any individual must be very minute. It must be 

 indifferent, therefore, whether that minute quantity which thus 

 acts, is a portion of a great body of the same substance, or 

 whether it is the whole which was generated ; and this must, in 

 reality, be indifferent in any case, even without transportation. 

 It is ascertained, that a single inspiration will produce the 

 disease; and, therefore, whether in this case, or in that of 

 dilution under transportation, the exposed person receives the 

 whole produce of a square yard, or any other given space, or 

 the same quantity out of a mass produced by twenty thousand 

 or a million of square yards, the consequence will be the same. 

 It is a mere question of arithmetic; and moreover, as the whole 

 malaria of any marsh is the collective sum of the portions pro- 

 duced by each plant or fragment, whatever can act on the 

 exposed individual will be sufficient to the effect, though it 

 were the entire quantity in existence, or the single part will 

 act as effectually as if it was accompanied by a thousand 

 similar ones. 



Thus it is of no consequence how small the marshy or per- 

 nicious spot is, provided it can act ; and the only difference is, 

 that a smaller spot will act on fewer persons ; and also, that 

 from the greater dilution which it must undergo on trans- 

 portation, it must act at shorter distances, or may require abso- 

 lute proximity or contact. 



Having premised these two necessary considerations, it is 

 sufficient now to inquire what are the modifications and forms of 

 soil or water, or of certain spots and places generally, which 



