On the Efficacy of Paragreles. 39 



There is an objection, which at once presents itself, — that 

 after the water is decomposed, the succeeding flash of light- 

 ning, would cause the reunion of the gases, and thus coni" 

 pletely neutralize the effects of the previous decomposition. 

 If the gases remained stationary after their production, this 

 would certainly be the case : but the specific levity of the 

 hydrogen will cause it to ascend with great velocity through 

 the dense vapours which press upon it from all sides ; and 

 the oxygen will have a tendency to take the opposite direc- 

 tion. 



In our attempt to give a reason for the efficacy of Para- 

 greles in preventing the destructive effects of hail, we are not 

 sensible of having advanced any thing that is hypothetical. 

 The decomposition of water by the electric spark is too well 

 known to be called in question. The absorption of heat, 

 when a body changes from the solid to the liquid, or from 

 the liquid to the aeriform state, has been matter of common 

 observation, since- Jhe days of Dr. Black ; and that a great 

 depression of temperature, in the contiguous bodies, must re- 

 sult, is too obvious to be denied. The most questionable 

 thing perhaps is, whether this reduction of temperature would 

 be sufficient to freeze the drops of rain. A few words on 

 this point may be acceptable. 



The combustion of one pound of hydrogen gas evolves suf- 

 ficient heat to melt three hundred and twelve pounds of ice.* 

 This gas has entered into combination with eight pounds of 

 oxygen, and the heat evolved is the latent heat of the two 

 gases. If we take the weight of one hundred cubic inches 

 of hydrogen to be 2.119 grains, and that of one hundred cu- 

 bic inches of oxygen, 33.915 grains,! the above weight of 

 the gases will be equal to about one hundred and fifty-seven 

 cubic feet of hydrogen, and 78.5 of oxygen. These quan- 

 tities form 186.5 cubic inches of water. The decomposi- 

 tion therefore of 186.5 cubic inches of water will absorb as 

 much heat as is required to melt three hundred and twelve 

 pounds of ice : or, supposing the heat absorbed by melting 

 ice to be 1 40° F.J If the temperature of the drops of water 

 be 72°, the heat abstracted will reduce the temperature of 

 one thousand and ninety two pounds to the freezing point, 



* Biot, Traite de Physique, Tom. 4, p. 704. (A Paris, 1816.) 



| Thomson's First Principles of Chemistry, Vol. 1, p. 71. (London, 1825.) 



X Thomson's Principles of Chemistry, Vol. 1, pp. 87,88. (Philadelphia, 1818.) 



