NATURAL PHILOSOPHY. 141 



cumuli of our latitudes. The -warmed air, charged with vapor, rises in 

 columns, so as to penetrate the vapor screen which hugs the earth ; in 

 the presence of space, the head of each pillar wastes its heat by radia- 

 tion, condenses to a cumulus, which constitutes the visible capital of an 

 invisible column of saturated air. 



Numberless other meteorological phenomena receive their solution, 

 by reference to the radiant and absorbent properties of aqueous vapor. 

 It is the absence of this screen, and the consequent copious waste of 

 heat, that causes mountains to be so much chilled when the sun is with- 

 drawn. Its absence in Central Asia renders the winter there almost 

 unendurable ; in Sahara the dryness of the air is sometimes such, that 

 though during the day " the soil is fire and the wind is flame," the chill 

 at night is painful to bear. In Australia, also, the therrnornetric range 

 is enormous, on account of the absence of this qualifying agent. A 

 clear day, and a dry day, moreover, are very different things. The 

 atmosphere may possess great visual clearness, while it is charged with 

 aqueous vapor, and on such occasions great chilling cannot occur by 

 terrestrial radiation. Sir John Leslie and others have been perplexed 

 by the varying indications of their instruments on days equally bright 

 but all these anomalies are completely accounted for by reference to 

 this newly-discovered property of transparent aqueous vapor. Its 

 presence would check the earth's loss ; its absence, without sensibly 

 altering the transparency of the air, would open wide a door for the 

 escape of the earth's heat into infinitude. 



DYNAMICAL THEORY OF HEAT. 



Professor Frankland, in a recent lecture before the Royal Institution, 

 London, presented the following points in reference to the dynamical 

 theory of heat : The amount of heat necessary to raise the temperature 

 of a body through a given number of degrees (e. g. from 32deg. to 212 

 deg.) is termed " the specific heat " of that body, and that an atom of 

 each solid element requires the same quantity of heat to raise its tem- 

 perature through the same number of degrees. Hence, at any given 

 temperature, the amount of heat-force associated with each solid element- 

 ary atom is the same ; but the proportion of this force evolved during 

 chemical combination differs in each element (which was shown experi- 

 mentally in the case of heated balls of lead and iron placed on cakes of 

 wax ; (he iron dissolving more wax than the lead). It was stated that 

 the greater the amount of heat evolved during combination, the more 

 difficult is the compound to decompose ; and it was shown that even 

 when atoms of the same kind are combined, heat is liberated. This oc- 

 curs, also, whenever alcohol and water are mixed, when paper is moist- 

 ened, etc. The heat-force associated with the atoms of matter exists as 

 molecular motion. When two or more atoms unite or come into col- 

 lision, a certain amount of this motion is destroyed and takes the form 

 of heat. The greater the amplitude of the molecular motion of two 

 bodies, in- so-called contact with each other, the more imminent is the 

 collision of their atoms. An augmentation of temperature increases 

 the amplitude of this molecular motion ; hence, heat usually promotes 

 chemical combination. In some cases, the molecular motion of two 

 bodies, at ordinary temperatures, is sufficient to bring them into col- 

 lision ; hence, what is termed " spontaneous combustion " (e. g., phos- 



