On Ni(/ht- Frosts. 25 



This qiiantity of water lias turned into ice and its temperatm-e has gone 

 doAvn to — 6". 



How much heat is required to raise tlie température of this quautity of 

 water to 0" and to liquify it at the same température, as well as to raise the 

 température of the air to 0"? 



In order to raise the température of GOO kg of water from -fi" to 0" 



a quantity of heat is required — 3,600 Cal. 



and to liquify the same quantity of water . = 48,000 „ 



The whole quantity of air in 10 hectares with 

 a height of 2 m amounts to 200,000 m^ the 

 average température of which is - 2^,5 and the 

 specific heat of the air being 0,2*7, we get 



0,247 . 2,5 . 200,000 ■ ■ = 123,500 Cal. 



or total 175,100 Cal. 



Coal, when hurning gives 8,000 Cal. per kg. Hence, the estimated 

 quantity of heat very nearly corresponds to 22 kg of coal, or 2,2 kg per 

 hectare. 



If we présume the heat, lost by the plants, to correspond with a layer of 

 ice 0,0001 m. in thickness, which is the same tliing as if the upper side of 

 every plant had been frozen in a layer of O,!"", we shaU get to 10 hectares 

 10 m' of ice, or 10,000 kg, which further require 10,000 . (80 ^ 6) = 

 860,000 Cal. or about 108 kg of coal. This quantity of heat exceeds, by 

 far, the preceding one and form with it a sum of about 130 kg of coal. 

 With this quantity of heat the température of the field would be raised to 0" 

 and the water liquid. Sliould the water retain its solid form, the required 

 quantity ol heat will diminish with no less than 



848,000 

 and hence there will remain only 



187,100 

 corresponding to 23,4 kg. of coal. 



But as plants can support température of —2° near the ground without 

 being damaged, the necessary quantity of heat will diminish still further to 



67,200 

 corresponding to 8,4 kg of coal. 



