On Night- Frosts. 69 



or 9,9 gr = 0,0099 kg pro m'. 



and thus to 0,2 m'. 



0,00198 kg. 



Whcn tlic vapoiir rcaches tho surface of thc carth, the lensioii, will havc fallen to 7,7 

 mm and thc numbcr of kg in the layer of eartli, to 



0,00156 kg 



and bcfore leaving thc surface of thc carth, thc tension will havc fallen to 5,4, corrcs- 

 ponding to 



0,ooiu kg 

 which gives a réduction of 



0,00084 _ 

 0,00198 



of thc wholc quantity. 



Out of onc kg 421 g havc thus been Condensed, corresponding to a quantity of heat of 



258,6 Cal. 



Thc remainder of thc cntirc quantity of heat, 609,6 contained in thc aq. vapour, or 



351,0 Cal. 



bas bccn carricd away by thc aq. vapour and is to be found absorbcd in it. 



If we observe tliis aq. vapour abovc the surface of thc carth during a night of frost, 

 wc shall find, Ihat, on rising, it will meet layers of air, thc température of which is higher; 

 its tension must, consequently, be increased and iustcad of radiating heat, this vapour will 

 become a consumer of it. 



As there are no expérimental data, we must content ourselves with having shown that 

 the cause of the loss of heat from the uppermost layer of earth is not to be found in ra- 

 diatio7l, hut chiefly in evaporation. It is clear that, during a frost night, this heat isyof 

 very littlc benefit to those parts of vegetation which are above thc ground. It seems however 

 obvions that it must have great influence on the roots of plants, and, at the same tinie, a 

 constant conveyance of water, frora the lower layers of earth to the higher, takes place, as 

 long as the surface of the earth is colder than the lower layers. Should the surface of the 

 earth be warmer, the heat is transported with the aq. vapour, which then is not Condensed. 



A striking évidence of the correctuess of the above e.^planation will be found in the 

 experiments made for determing the quantity of dew. These have bcen made by means of 

 a very, dew-forming apparatus. 



If, for instance, a pièce of wadding is placed on the surface of the earth the cooling 

 of that part of the surface which is immediatoly under it, is prevented, inasmuch as both 

 radiation and conducting of heat are broken off and the aq. vapour from the upper layer 

 of earth is Condensed only in the wadding. It is evident that thc quantity of dew of the 

 latter must increase considei'ably by this process. Using such data, as starting-points for the 

 détermination of this quantity, would be decidedly wrong. 



