218 Scientific Intelligence. — Chemistry. 



stone which is nearly dry ; and most Ume-burners are well ac- 

 quainted with this fact, and sprinkle with water the limestone 

 which has been long procured before they charge the kilns with 

 it." — (Dumas, Traite de Chimie, ii. p. 482.) The first of these 

 explanations is, however, inadmissible, for hydrate of lime is 

 decomposed at a temperature lower than that at which carbon- 

 ate of lime is decomposed vmder the influence of water. On 

 considering the circumstances of the combustion in limekilns, the 

 second explanation does not appear to M. Gay I.ussac to be ap- 

 plicable, and he therefore proceeds to some observations which 

 he thinks will explain the influence of the water. A porcelain 

 tube was filled with bits of marble and placed in a furnace, the 

 heat of which was easily regulated ; a glass retort containing wa- 

 ter was adapted to one end of the tube, and at the other a glass 

 tube to receive the carbonic acid gas. The heat was raised suf- 

 ficiently high to decompose the marble, and on shutting the ash- 

 pit door the heat fell to a dull red, and the carbonic acid ceased 

 to come over ; and at this instant the water was boiled in the re- 

 tort, and carbonic acid was abundantly obtained. On discontinu- 

 ing the vapour, the disengagement of acid instantly ceased, and 

 returned only on continuing the vapour. It appears, therefore, 

 proved that the vapour of water actually favours the decomposi- 

 tion of the carbonate of lime by heat, and that by its operation 

 this decomposition may occur at a lower temperature than is 

 otherwise requisite. M. Gay-Lussac considers the action of the 

 water to be entii'ely mechanical. When the carbonate of lime is 

 exposed to heat, and is near the point of decomposition, an at- 

 mosphere of carbonic acid is formed around it, which presses up- 

 on the acid remaining combined, 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 tem- 

 perature, or removing the carbonic acid and forming a vacum, 

 or by displacing it, either by the vapour of water, or some other 

 elastic fluid, such as atmospheric air. This explanation is sup- 

 ported by the following experiment : Carbonate of lime was 

 heated in a porcelain tube nearly to its decomposing point, and 

 then a current of atmospheric air was passed over it. The dis- 

 engagement of carbonic acid immediately commenced, continued 

 with the current of air, ceased when it was stopped, and recotn- 



