90 FOURTH REPORT — 1834. 



Pontine marshes, covering an area of eight leagues by two, have 

 spread their deleterious exhalations, in certain directions of the 

 wind, to the mouth of the Tiber. In the West Indies miasms 

 have affected the crews of vessels mooi-ed 1500 toises (3200 

 English yards) from the shore (Monfalcon) ; but this is pro- 

 bably much more than the usual distance. 



3. The chemical properties of marsh miasms have been in- 

 vestigated by several writers, but with little other fruit than a 

 catalogue of negative qualities*. Miasms are not the mere pro- 

 ducts of putrefactionf, and have not necessarily a fetid odour. 

 Experiment has not demonstrated any departure, in the air over 

 marshes, from its usual proportions as to oxygen and azotic 

 gases. Neither carburetted, sulphuretted, or phosphuretted hy- 

 drogen, nor ammonia, has been detected in these exhalations. 

 The principle on which their peculiar agency depends, still re- 

 mains to be determined by experiment. 



4. There are several points of analogy between the operation of 

 marsh miasms, and that of contagious poisons, upon the human 

 body. Both require a certain predisposition in the persons ex- 

 posed to them ; and this susceptibility is imparted by the same 

 causes. The power of resisting miasms as well as contagions is 

 acquired by habit, at least to a certain exjtent. But no continu- 

 ance of usage ever protects persons, who are constantly exposed 

 to an atmosphere impregnated with exhalations constituting 

 malaria, from their pernicious effects. In some marshy coun- 

 tries, the perfect and vigorous human form is never seen ; and 

 a race of men inhabit them who are alike destitute of physical 

 and mental energy, and who in middle life exhibit all the signs 

 of old age. Strangers arriving there are doomed to inevitable 

 destruction ; and all attempts to extend our geographical know- 

 ledge of such regions, however well concerted, have been baffled 

 by the overwhelming power of endemic pestilence. 



XLI. Epidemics are much more widely diffused than ende- 

 mics ; so widely, indeed, that they have been imputed to certain 

 conditions of the atmosphere, called epidemic constitutions of the 

 air. To this term there can be no objection, provided it involve 

 no hypothesis as to causes. The only legitimate meaning of the 

 word epidemic is, an acute disease prevailing over the ivhole or 

 a large portion of a community, at seasons not in general 



* The most elaborate and able work which I have seen on the subject, is the 

 Recherches Historiques, Chimiques, et Aledicales sur I'Air Marecageux, par 

 J. S. E. Julia. 8vo. Paris, 1823. 



f It has been suggested {Foreign Quarterly Review, No. XXI.) that miasms 

 are the products of plants of the genus Chara. 



