320 DISCOURSE ON THE STUDY 



cations coincide exactly with that of the air-thermo- 

 meter in moderate temperatures ; though at very 

 elevated ones they exhibit a sensible, and even 

 considerable, deviation. By this instrument, which 

 owes its present convenience and utility to the 

 happy idea of Newton, who first thought of fixing 

 determinate points on its scale, we are enabled to 

 estimate, or at least identify, the degrees of heat ; 

 and thereby to investigate with accuracy the laws 

 of its communication and its other properties. Were 

 we sure that equal additions of heat produced equal 

 increments of dimension in any substance, the indi- 

 cations of a thermometer would afford a true and 

 secure measure of the quantity present. ; but this is 

 so far from being the case, that we are nearly in 

 total ignorance on this important point ; a circum- 

 stance which throws the greatest difficulty in the way 

 of all theoretical reasoning, and even of experi- 

 mental enquiry. The laws of the dilatation of liquids, 

 in consequence of this deficiency of necessary pre- 

 liminary knowledge, are still involved in great ob- 

 scurity, notwithstanding the pains which have been 

 bestowed on them by the elaborate experiments and 

 calculations of Gilpin, Blagden, Deluc, Dalton, Gay- 

 Lussac, and Biot. 



(357.) The most striking and important of the 

 effects of heat consist, however, in the liquefaction of 

 solid substances, and the conversion of the liquids 

 so produced into vapour. There is no solid substance 

 known which, by a sufficiently intense heat, may 

 not be melted, and finally dissipated in vapour ; and 

 this analogy is so extensive and cogent, that we can- 

 not but suppose that all those bodies which are 



