160 LECTUKES ON 



the air especially, heat radiates without being scarcely absorbed. 

 The soil and solid bodies become warmed according to their individ- 

 ual capacity, and from them the air receives the heat which warms 

 it. Ffom the moist surface of the soil goes on a rapid evaporation, 

 which renders latent* a large amount of heat, so that the tempera- 

 ture of the soil is not rapidly but gradually elevated. The ascent of 

 water from the sub-soil to supply the place of that evaporated goes 

 on as before described. The liquid water of the soil has combined 

 with (rendered latent) a vast amount of heat therefrom, and passed as 

 o-aseous water (vapor) into the air. When the sun declines, the process 

 diminishes in intensity, and when it sets, the reverse takes place. 

 The heat that had accumulated on the surface of the earth radiates 

 into the cooler atmosphere and planetary space, the temperature of 

 the surface rapidly diminishes, and the air itself becomes cooler by 

 convection, t As the cooling goes on, the vapor suspended in the at- 

 mosphere begins to condense upon cool objects, while its latent heat 

 becoming free hinders the too sudden reduction of temperature. The 

 condensed water collects in drops — it is dew; or in the colder seasons 

 it crystallizes as hoar-frost. 



The special nature of the surface of the soil is closely connected 

 with the maintenance of a uniform temperature, with the prevention 



* When a piece of ice is placed in a vessel whose temperature is increasing, hy means of 

 a lamp, at the rate of one degree of the thermometer every minute, it will be found that 

 the temperature of the ice rises until it attains 32°. When this point is reached, it be- 

 gins to melt, but does not suddenly become fluid : the melting goes on very gradually. A 

 thermometer placed in the water remains constantly at 32° so long as a fragment of ice 

 is present. The moment the ice disappears, the temperature begins to rise again, at 

 the rate of one degree per minute. The time during which the temperature of the ice 

 and water remains at 32° is 140 minutes. During each of these minutes one degree of 

 heat enters the mixture, but is not indicated by the thermometer — the mercury remains 

 stationary; 140° of heat have thus passed into the ice and become hidden, latent; at the same 

 time the solid ice has become liquid water. The difference, then, between ice and water 

 consists in the heat that is latent in the latter. If we now proceed with the above experi- 

 ment, allowing the heat to increase with the same rapidity, we find that the temperature 

 of the water rises constantly for 180 minutes. The thermometer then indicates a temper- 

 ature of 212, (32+180,) and the water boils. Proceeding with the experiment, the water 

 evaporates away, but the thermometer continues stationary so long as any liquid remains. 

 After the lapse of 972 minutes, it is completely evaporated. Water in becoming steam 

 renders, therefore, still another portion, 972°, of heat latent. The heat latent in steam is 

 indispensable to the existence of the latter. If this heat be removed by bringing the 

 steam into a cold space, water is reproduced. If, by means of pressure or cold, steam be 

 condensed, the heat originally latent in it becomes sensible, free, and capable of affecting 

 the thermometer. If, also, water be converted into ice, as much heat is evolved and made 

 sensible as was absorbed and made latent. It is seen thus that the processes of liquefac- 

 tion and vaporization are cooling processes; for the heat rendered latent by them must be 

 derived from surrounding objects, and thus these become cooled. On the contrary, solidi- 

 fication, freezing, and vapor-condensation are warming processes, since in them large quan- 

 tities of heat cease to be latent and are made sensible, thus warming surrounding bodies. 



-j- Though liquids and gases are almost perfect non-conductors of heat, yet it can diffuse 

 through them rapidly, if advantage be taken of the fact that by heating they expand and 

 therefore become specifically lighter. If heat be applied to the upper surface of liquids or 

 gases, they remain for a long time nearly unaffected; if it be applied beneath them, the lower 

 layers of particles become heated and rise, their place is supplied by others, and so currents 

 upward and downward are established, whereby the heat is rapidly and uniformly distrib- 

 uted. This process of convection can rarely have any influence in the soil. What we have 

 stated concerning it shows, however, in what way the atmosphere may constantly act in 

 removing heat from the surface of the soil. 



