38 Influence of Atmospheric Moisture 



do not occur in the hygrometrical state of the air, without cor- 

 responding changes in its temperature, weight, and electrical 

 condition. Even the common fog, or mist, is far from being 

 a single or simple phenomenon. In some instances it is the 

 cloud already formed, and brought by currents of air or other 

 causes to a lower level ; — in other instances, as in the fogs 

 which occasionally intervene between thunder-storms, the re- 

 sult apparently of a change going on in the electrical relations 

 of the earth and atmosphere at the spot, producing alterations 

 in the hygrometrical state of the latter. Science has not yet 

 assigned their proper place to these several changes, as regards 

 the relation of cause and effect. But however this be deter- 

 mined hereafter, the complex nature of their action on the 

 body still remains, and wiU long retard any certain conclusions 

 on the subject. 



Another source of ambiguity, in considering the effects of 

 different degrees of humidity of the air, is the influence of local 

 circumstances of soil and surface in modifying this ; especially 

 in that lower stratum of the atmosphere, with which man has 

 chief concern : — and this modification regards not merely the 

 quantity of water taken up by the air, or precipitated from it, 

 according to the several conditions of the surface, and the 

 action of external sources of temperature ; but also the various 

 miasmata disengaged, or otherwise acted upon, by the same 

 processes. I have already adverted to these material causes 

 of disease, in their more particular relation to heat. What- 

 ever their nature (and we have every thing still to learn here), 

 it seems certain that the presence of moisture, either on the 

 surface or near it, under the form of vegetation, damp air or 

 soil, and acted upon by a certain degree of temperature, con- 

 tributes much to their production, if not indeed essential to 

 it. And to these conditions, conjoined perhaps with the elec- 

 trical state of the atmosphere, we may chiefly attribute the 

 greater unhealthiness of the rainy seasons in tropical climates, 

 which the mere quantity of rain falling will not equally ex- 

 plain. 



But further than this, there is some cause to presume that 

 aqueous vapour in the atmosphere, whatever its mode of com- 

 bination, i^ much concerned in giving activity and spread to 



