OF NATURAL PHILOSOPHY. 319 



state into vapour, which is the most powerful source 

 of cold known. 



(355.) The dilatation of bodies by heat forms the 

 subject of that branch of science called pyrometry. 

 There is no body but is capable of being penetrated 

 by heat, though some with greater, others with less 

 rapidity ; and being so penetrated, all bodies (with a 

 very few exceptions, and those depending on very pe- 

 culiar circumstances,) are dilated by it in bulk, though 

 with a great diversity in the amount of dilatation 

 produced by the same degree of heat. Of the several 

 forms of natural bodies, gases and vapours are ob- 

 served to be most dilatable ; liquids next, and solids 

 least of all. The dilatation of solids has been made 

 a subject of repeated and careful measurement by 

 several experimenters; among whom, Smeaton, 

 Lavoisier, and Laplace, are the principal. The 

 remarkable discovery of the unequal dilatation of 

 crystallised bodies by Mitscherlich has already 

 been spoken of. (266.) That of gases and vapours 

 was examined about the same time by Da] ton and 

 Gay-Lussac, who both arrived independently at the 

 conclusion of an equal dilatability subsisting in 

 them all, which constitutes one of the most remark- 

 able points in their history. 



(356.) The dilatation of air by heat affords, per- 

 haps, the most unexceptionable means known of 

 measuring degrees of heat. The thermometer, as 

 originally constructed by Cornelius Drebell, was an 

 air thermometer. Those now in common use 

 measure accessions of heat not by the degree of 

 dilatation of air but of mercury. It has been shown, 

 by the researches of Dulong and Petit, that its indi- 



