170 WRITINGS OF JOSEPH HENRY. [1855- 



from this cause. It is probable that the diffusion of malaria 

 in still air, as in the case of vapor, is exceedingly slow, and 

 hence anything that tends to interrupt the current will much 

 retard its transmission. It is asserted that in some cases 

 near the focus of emanation it is less deleterious than at 

 places at a considerable distance. It would appear from 

 this to ascend vertically with the columns of heated air and 

 to be afterwards wafted horizontally to a distance, and there 

 impinging on the first elevation produces its effects; or per- 

 haps this opinion has arisen from the screening influence of 

 objects near the source. 



Miasma in perfectly dry air is in such small quantities as 

 not only to be inaccessible to the investigation of science, 

 but also insufficient to seriously affect human life. It is 

 otherwise however in air cooled by the radiation of the even- 

 ing and night. It appears then to be precipitated into the 

 lower strata of the atmosphere with the mass of humidity 

 with which it is probably connected, and when this is again 

 evaporated at sunrise, it carries up with it the miasma in 

 its ascending movement. At this time it is taken into the 

 system by swallowing, respiration, and possibly by absorp- 

 tion through tlie pores of the skin, in sufficient quantities 

 to manifest its deleterious effects. In malarious districts 

 therefore caution should be taken against exposure to the 

 evening precipitation and morning evaporation of the hu- 

 midity of the atmosphere. Ground which has been a long 

 time under water retains during a series of years the prop- 

 erty of emitting the effluvia. The virgin soil in which decay- 

 ing vegetable matter has accumulated for years, when first 

 exposed to the action of the air by the labor of the pioneer? 

 gives off a large amount of malarious effluvia ; care should 

 therefore be taken in the settlement of a new country not 

 only to select a proper location, but also to protect the houses 

 b}"" a border of trees, particularly on the side against which 

 the prevailing wind impinges. And it is to be regretted that 

 good taste, as well as the comfort of an agreeable shade, does 

 not more frequently induce the husbandman to spare some 

 of the original products of the forest which are found near 



