-1859] WRITINGS OF JOSEPH HENRY. 171 



the spot on which he erects his dwelling. It is also stated 

 that plants in active vegetation, as in the case of sunflowers, 

 absorb deleterious effluvia ; but whether this effect is pro- 

 duced independently of the screening we have mentioned 

 has not yet been settled. In the fertile regions of the tropics 

 where heat and moisture abound — for example, the valley 

 of the Amazon — and where vegetation is luxuriant, the mala- 

 rious effluvium is at its maximum; while in dry countries 

 with less vegetable life, such as those west of the Mississippi, 

 it is not found. Nature thus is not indiscriminately benev- 

 olent to civilized man ; in his uncivilized condition different 

 races are confined to different districts, and the influences 

 which affect one are inoperative on the other. It is only by 

 investigating the causes of these differences, and thus in 

 some cases arriving at the means of controlling them, that 

 the civilized man becomes a citizen of the world, and within 

 certain limits is enabled to overcome the natural enemies to 

 which in his primitive ignorance he is exposed. 



The difficulty of investigating the nature of miasma has 

 induced some to believe its effects due to variations of tem- 

 perature and moisture ; but this is not sufficient to explain 

 all the phenomena, as places very different in this respect 

 vary greatly in their sanitary condition. The quantity of 

 material (whatever it may be) which constitutes malaria is 

 too minute to be immediately detected by the eudiometer, 

 the instrument usually employed to analyze air. M. Moscati, 

 in order to collect it in considerable quantities, employed a 

 glass globe filled with ice, on the surface of which the aqueous 

 vapor of the atmosphere was constantly precipitated. He 

 found that the water thus collected in infected places was of 

 a white color, inodorous, slightly alkaline, and after stand- 

 ing a short time lime-water and acetate of lead produced in 

 it a light precipitate. It contained animal matter, ammonia, 

 and chlorate and carbonate of soda. The effect of this water 

 upon animals has not (so far as we know) been tested, 

 though it is said that sheep which feed upon grass covered 

 by the morning dew in infected districts are subject to pecu- 

 liar maladies. 



