240 WRITINGS OF JOSEPH HENRY. [1855- 



it contains, we may discuss some of the minor physical phe- 

 nomena connected with the process of evaporation and the 

 existence of water in an aeriform condition. 



Heat and moisture are the principal essential atmospheric 

 agents in the production of vegetable matter, and where 

 these are not found in sufficient quantities, however rich 

 may be the soil in fertilizing materials, at least comparative 

 if not absolute sterility must prevail. Unfortunately how- 

 ever, these conditions though so highly favorable to the 

 production of the substances which administer to the neces- 

 sities and conveniences of life, are not equally favorable to 

 the condition of health of the more highly civilized races of 

 men. Heat and moisture are also the essential conditions 

 under which the deadly malarious effluvia exert their bane- 

 ful influence — especially upon the white race ; and though 

 science may hereafter furnish the means of disarming them 

 of their terrors, yet at present they require the rich harvests 

 of fields which would otherwise be uncultivated, to be reaped 

 by the labor of individuals of another race so different in 

 their physical organization as to be apparently exempt from 

 the effects of these aerial poisons. The fertile rice, cotton, 

 and sugar fields of the southern portion of the United States, 

 are cultivated by negroes not only with impunity but with- 

 out impairment of their physical enjoyments of life. 



The relative moisture of different countries is intimately 

 connected with their condition as to healthfulness. While 

 in the moist climate of Great Britain and that of some of the 

 West India islands diseases of the lungs are prevalent, they 

 are seldom known in the dry regions of Nebraska and Min- 

 nesota, 



From the experiments of Dalton, as we have seen, the 

 rapidity of evaporation is proportional to the difference of 

 elastic tension of the vapor in the air and that of the evapo- 

 rating surface. Meteorologists have generally adopted as the 

 expression of relativehumidity the ratio of the force of vapor in 

 the air to the force which it would have were it perfectly satu- 

 rated, or theysometimesadoptan equivalent expression by de- 

 fining the relative humidity to be the ratio of the absolute 



