HEAT. 



nature of heat, " a subject," says he, 

 " upon which the popular opinion seems 

 to have been lately led away by very su- 

 perficial considerations. The facility 

 with which the mind conceives the exist- 

 ence of an independent substance, liable 

 to no material variations, except those of 

 its quantity and distribution, especially 

 when an appropriate name, and a place 

 in the order of the simplest elements 

 has been bestowed on it, appears to 

 have caused the most eminent chemical 

 philosophers to overlook some insupe- 

 rable difficulties attending the hypothe- 

 sis of caloric." In another part of the 

 same passage he remarks, that " the cir- 

 cumstances which have already been 

 stated, respecting the production of heat 

 by friction, appear to afford an unan- 

 swerable confutation of the whole of this 

 doctrine. If the heat is neither received 

 from the surrounding bodies, which it 

 cannot be, without a depression of their 

 temperature ; nor derived from the quan- 

 tity already accumulated in the bodies 

 themselves, which it could not be, even 

 if their capacities were diminished in 

 any imaginable degree ; there is no alter- 

 native, but to allow that heat must be 

 actually generated by friction : and if it 

 is generated out of nothing, it cannot be 

 matter, nor even an immaterial or semi- 

 material substance. The collateral parts 

 of the theory have also their separate 

 difficulties : thus, if heat were the gene- 

 ral principle of repulsion, its augmen- 

 tation could not diminish the elasticity 

 of solids and of fluids ; if it constituted 

 a continued fluid, it could not radiate 

 freely through the same space in diffe- 

 rent directions ; and if its repulsive par- 

 ticles followed each other at a distance, 

 they would still approach near enough 

 to each other, in the focus of a burning- 

 glass, to have their motions deflected 

 from a rectilinear direction." 



In page 656, the same author, having 

 drawn a parallel between the production 

 of heat and sound, observes that " all 

 these analogies are certainly favourable 

 to the opinion of the vibratory nature of 

 heat, which has been sufficiently sanc- 

 tioned by the authority of the greatest 

 philosophers of past times, and of the 

 most sober reasoners of the present. 

 Those, however, who look up with un- 

 qualified reverence to the dogmas of the 

 modern schools of chemistry, will, pro- 

 bably, long retain a partiality for the 

 convenient, but superficial and inaccu- 

 rate, modes of reasoning, which have 



been founded on the favourite hypothe- 

 sis of the existence of caloric as a sepa- 

 rate substance ; but it may be presumed 

 that, in the end, a careful examination 

 of the facts, which have been adduced 

 in confutation of that system, will make 

 a sufficient impression on the minds of 

 the cultivators of chemistry, to induce 

 them to listen to a less objectionable 

 theory." The question, therefore, re- 

 mains undetermined ; and it is fortunate 

 that, most of the phenomena connected 

 with the operation of heat, may be ex- 

 plained equally well upon either theory. 



CHAPTER III. 

 Of the Expansion of Bodies by Heat. 



THE most general effect produced by 

 heat upon bodies to which it is applied, 

 is the enlargement of their bulk. Solids, 

 fluids, and airs, all expand on being 

 heated, and contract when they are 

 cooled. Some useful processes of art 

 and several important operations of na- 

 ture depend upon this law. 



The expansion of solids may readily 

 be proved by simple and convincing 

 experiments. 



Ex. A cylindri- 



cal piece of brass 

 a,(y?g-.l,)havinga 

 handle adapted to 

 it, is fitted to a flat 

 piece b, so that it 

 may just pass 

 through the large 

 notch lengthwise, 

 and by its ends 

 go through the 

 round hole : when 

 heated in the fire 

 it will be too long 

 to pass in one di- 

 rection, and too 

 thick to pass in 

 the other. Hav- 

 ing become cold, it will again fit and 

 pass through, as before. 



Ex. An iron ball, adapted to a ring 

 of the same metal, so as to pass through 

 when cold, will be too large to pass 

 when heated; when cooled again it 

 will pass as before. 



If the relative degrees of expansion 

 which different bodies undergo at low 

 temperatures, are to be ascertained, the 

 instrument (fig. 2) called a pyrometer, 

 or some similar one must be used. 



Ex. A rod of any metal or other 



