TEMPERATURE, 153 



during the former period, certain substances which do not seem 

 to exist therein during the latter, so that the disengagement, or at 

 least the condensation, of marsh emanations is less during the 

 day, and greater at night, but particularly at sunset and sunrise. 

 Their chemical nature has not yet been ascertained by analysis ; 

 however, they have been found to contain ammoniacal com- 

 ponents, and, therefore, must be of an organic or vegeto-animal 

 nature. No doubt their mode of operating is complex ; but in 

 order to account for the identical effects produced by the atmo- 

 sphere of swampy places, and from the consideration of the pro- 

 cesses of vegeto-animal decay which are uninterruptedly going on 

 in such localities, we are led to admit that they give rise to cer- 

 tain noxious principles, which we call effluvia, miasma, and malaria. 

 Now, are these substances the elements of the noxious influence 

 of swamps ? This would appear to be highly probable. 



The specific gravity of effluvia seems to be greater than that 

 of pure air, for they do not spread beyond a certain height. It 

 would seem, also, that they are soluble in the globular moisture, 

 actually saturating the air, so that they become condensated by 

 coolness of the night, and fall with the dew the popular dread 

 of dew-damps being proved thereby to be well founded. 



Local circumstances may greatly influence the condensation 

 of effluvia; for instance, whenever a swamp exists in a deep 

 valley, abuts on a hill or mountain, or is skirted by forests, the 

 miasmas evolving therefrom are likely to accumulate in the 

 locality. The direction of variable or prevailing winds, also, aids 

 materially in the spreading of malaria, and may thus modify the 

 salubrity of a whole district. 



Hills, mountains, and forests, may therefore act as a protec- 

 tion against, or, on the contrary, as an aggravation of the noxious 

 influence of a swamp, according as it is situated to the leeward or 

 windward of the tract. In fact, it often happens that the 

 miasmatic influence is almost null in the locality of the swamp 

 itself, whilst it becomes very noxious at a certain distance, particu- 

 larly whenever any barrier obtrudes in the shape of hills and moun- 

 tains to the leeward. Localities to the windward are compara- 

 tively exempt and safe. 



The formation of effluvia is more abundant when a larger sec- 

 tion of a marsh remains uncovered, or exposed to the direct action 

 of the solar rays ; so that, in our island, the deleterious influence 



