Intelligence and Miscellafieous Articles, 497 



It appears, therefore, proved that the vapour of water actually 

 favours the decomposition of the carbonate of lime by heat, and 

 that by its operation this decomposition may occur at a lower tem- 

 perature than is otherwise requisite. 



M. Gay-Lussac considers the action of the water to be entirely 

 mechanical. When the carbonate of lime is exposed to heat, and 

 is near the point of decomposition, an atmosphere of carbonic acid 

 is formed around it, which presses upon the acid remaining com- 

 bined, so that the latter, that it may be disengaged, must overcome 

 the pressure of this atmosphere. This, however, cannot occur 

 without still further raising the temperature, or removing the car- 

 bonic acid and forming a vacuum, or by displacing it, either by the 

 vapour of water, or some other elastic fluid, such as atmospheric 

 air. 



This explanation is supported by the following experiment; car- 

 bonate of lime was heated in a porcelain tube nearly to its decom- 

 posing point, and then a current of atmospheric air was passed over 

 it. The disengagement of carbonic acid immediately commenced, 

 continued wiih the current of air, ceased when it was stopped, and 

 recommenced with it. 



M. Gay-Lussac, therefore, considers it as proved, that the influ- 

 ence of aqueous vapour, in the calcination of limestone, is confined 

 to the production of a vacuum for carbonic acid, and to the pre- 

 vention of the pressure of the disengaged acid upon that which re- 

 mains with the lime. When the vapour is present, a lower tem- 

 perature is required to dislodge the carbonic acid ; but the impor- 

 tance of this influence must not be overrated. The water, in calcare- 

 ous stones, is mechanically interposed between their particles ; and 

 with the exception of some minute portions, which remain confined 

 in the centre of pieces too large to allow of the heat penetrating 

 and vaporizing them, the greater part of the water must evaporate 

 without any useful result ; and, on the contrary, with the loss of 

 fuel, before the limestone has acquired the temperature requisite 

 for its decomposition. 



M. Gay-Lussac thus concludes : " I am certainly convinced that 

 the vapour of water favours the calcination of limestone, but I am 

 doubtful as to its possessing real advantages, because there is not 

 a great difference in the temperature at which it decomposes, alone 

 or with the vapour of water : besides, if the vapour of water only 

 exerts a mechanical action similar to that of atmospheric air, any 

 important advantage which it possesses over the aeriform current 

 continually passing over the burning mass, is not evident. 



The readier decomposition of carbonate of lime by the access 

 of aqueous vapour, or rather by means of a vacuum, cannot be con- 

 sidered as an isolated fact. It may be regarded as an established 

 principle, that when one or several gaseous products are obtained, 

 either by the action of heat or a chemical agent, the decomposition 

 may be generally facilitated, by keeping the bodies in vacuo, or by 

 preventing the gaseous fluids from pressing upon it. And on the 

 other hand, the decomposition may be retarded or entirely pre- 



Third Series. Vol. 10. No. 6l3. June 1837. 3 S 



