24 



substances, when issuing out of the rocks in a melted state, must 

 be the produce of an incandescent nucleus. We have only three 

 primary divisions, viz. solid, fluid and gaseous : we have no in- 

 dependent igneous elements ; the latter division is merely the 

 effect of decomposition. Yet although this fact is so well known, 

 the igneous theory is still pertinaciously adhered to. 



The word heat generally implies the sensation which we ex- 

 perience on approaching a fire ; but in the sense it bears in 

 physics, it denotes the cause, whatever it be, of that sensation, 

 and of all the other phenomena which arise on the application 

 of fire, or of any other heating media. We should be greatly 

 deceived if we referred only to sensation as an indication of the 

 presence of this cause. Many of those things which excite in 

 our organs, and especially those of taste, a sensation of heat, 

 owe this property to chemical stimulants, and not at all to their 

 actual heat. 



There are a number of chemical agents which, from their cor- 

 roding, blackening, and dissolving or drying up the parts of some 

 descriptions of bodies, and producing on them effects not gene- 

 rally unlike those produced by heat, are said, in loose and vulgar 

 language, to burn them ; and this error has even become rooted 

 into a prejudice, by the fact that some of these agents are capa- 

 ble of becoming actually and truly hot during their action on 

 moist substances, by reason of their combination with the water 

 which the latter contain. 



Fire, or the combustion of inflammable bodies, is nothing 

 more than a violent chemical action attending the combination 

 of their ingredients with the oxygen of the air. 



One of the arguments brought forward in support of the ig- 

 neous theory, or central heat, is that it is found, by experiments 

 in mines, that the heat increases with the depth, and that hot 

 springs and mineral waters are found in all countries. Had this 

 increased temperature proceeded from the radiation of an incan- 

 descent nucleus, we should have a more uniform variation as we 

 descend. In South America, where a great number of experi- 

 ments were made, the variations in the degrees of heat were not 

 only very irregular and confined to particular patches in the 

 rocks as we descended, but also in some instances the tempe- 

 rature was found many degrees loiver at a greater depth than near 



