528 Mi- H. Meikle on the Specific Heat 



this gelatinous silica occurs. This position is very different 

 from that of the thermogenous quartz of the islands of Iceland 

 and Ischia. — Annales des Mines 1826. 



Experiments to compare the specific Heat of Air, under a con-- 

 stant volume, with its specific Heat under a constant pressure. 

 By Mr Hekry Meikle. (Communicated by the Author.) 



XT has been long known, that gaseous bodies emit heat when 

 compressed, and absorb it when dilated, — a property, by the by, 

 which is not easily reconcileable with the creed of those who sup- 

 pose heat to be mere motion. Little, however, was ascertained, 

 for a considerable time, regarding the amount of the change of 

 temperature accompanying a given change of density. The 

 earliest experiments to determine this question seem to have been 

 those of Professor Leslie. Mr Dalton and M. Gay Lussac have 

 also engaged in the same inquiry ^. Aa the heat evolved or ab- 

 sorbed by a change of density, depends on the difference between 

 the specific heat under a constant pressure, and that under a 

 constant volume, if we could find the ratio of these quantities, 

 we should be enabled to determine their relation to the heat 

 evolved or absorbed, and from this the change of temperature, 

 and conversely. From certain experiments of MM. Delaroche 

 and Berard, the Marquis de Laplace instituted some calcula- 

 tions -f-, which happened to come nearer the point than could 

 have been expected ; for these experiments were not at all suited 

 to the purpose ; and it is the more remarkable, that they should 



• According to the experiments of this last anthor, linder or amadou is in- 

 flamed by the sudden compression of air into one-fifth of its bulk. 'Some have 

 even questioned the fact, and others conjecture, that combustion commences 

 at lower temperatures, as the air is denser. But may we not suppose, with 

 more probability, that the pressure on the tinder, being suddenly augmented 

 in an almost nine-fold ratio, should elicit much heat from this compressible 

 substance itself? So that, till something else be known on the subject, we 

 need neither doubt the fact, nor believe that a fivefold compression of air 

 would of itself generate an inflammatory temperature. The melting of fine 

 wires, or thin metallic leaves, would afford a surer test of the temperature irt 

 compressed air, than the kindling of soft spongy bodies. 



*• Annales de Chimie et de Phys. iii. 238. 



