108 HEAT OR CALORIC. 



During hot weather, cold water, in almost any vessel, but soonest 

 in a metallic one, produces drops of condensed vapor upon the out- 

 side and a freezing mixture will generate hoar frost from the driest air. 



If the air were deprived entirely of moisture, it would, during res- 

 piration, parch the membranous lining of the passages, and thus 

 produce great inconvenience, and eventually serious mischief, in 

 breathing. 



(g.) Evaporation injures health by raising into the air miasmata, 

 produced by animal and vegetable putrefaction. This is too evident 

 to need illustration ; the effect is dependent on a certain degree of 

 heat, aided by moisture, as is seen in the rice swamps of our south- 

 ern states. Fever and ague* probably arise chiefly from this cause. 

 In cold countries extensive swamps do little or no mischief, and even 

 in those that are temperate, they are comparatively harmless. The 

 region about the river Sorel, in Lower, and the Welland Canal, in 

 Upper Canada, are examples. In particular seasons, however, such 

 countries become sickly. 



(h.) Evaporation supplies the moisture necessary to form rmn 1 

 snow, hail, hoar frost, dew, fogs, mist, fyc. This precipitation takes 

 place according to the state of the atmosphere ; it is much influenc- 

 ed by the mingling of currents of air, differing in temperature, and 

 in the quantity of vapor they contain. 



Precipitation of dew, hoar frost, &c. is much affected by radiation, 

 from the surface of the earth, and this depends greatly on the pre- 

 valence or absence of clouds. 



Radiation is most abundant in a clear night, when the temperature 

 of the ground is often several degrees lower than that of the air. 

 The frost is often caused, principally, by radiation from the ground ; 

 hence, it frequently freezes on the ground when the air is not as low 

 as 32. This subject has been fully illustrated by Dr. Wells, and he 

 has explained, why condensation of atmospherical vapor takes place 

 when there is not cold enough in the air to produce it ; it is because 

 the surfaces on which the vapor is precipitated, are colder than the 

 air ; those surfaces that radiate the best, will therefore be the coldest ; 

 hence, glass will be colder than metals. 



This radiation from the earth's surface is of the utmost importance 

 to vegetation, especially in hot climates ; plants radiate heat very 

 powerfully, and hence, they are often covered with dew, when the 

 naked ground is scarcely moist. This effect is much favored by 

 the clear, cloudless skies, of hot climates, while in colder regions, 

 there is more cloudy weather. The earth is there cold and damp and 



* Malaria is the classical word now applied to all such effects, and to their causee- 

 wbether understood or not. 



