HEAT. 



33 



The properties of different colours in 

 absorbing, reflecting, and radiating heat 

 are well worthy of attention in regu- 

 lating our summer and winter clothing. 

 Since dark colours have been proved by 

 the experiments of Dr. Franklin, Mr. 

 Leslie, Sir H. Davy, and others, to ab- 

 sorb heat in a much greater degree 

 than the lighter colours, it does not 

 seem advisable to use dark-coloured 

 clothes, particularly black, as summer 

 clothing, on account of the power that 

 such dark colours have of absorbing 

 the heat of the sun's rays. Since those 

 surfaces that absorb heat most abun- 

 dantly, also radiate most freely, the 

 propriety of using dark-coloured clothes 

 in winter, may admit of being ques- 

 tioned. 



If, instead of the heated body in the 

 arrangement shown at (fig. 1 0) we sub- 

 stitute a mass of ice, the effect will be 

 the reverse of what it was before, the 

 temperature of the thermometer being 

 rapidly reduced : this used to be called 

 the radiation of cold, and was ascribed 

 to the effect of a frigorific or cooling 

 principle, the existence of which has 

 long ceased to be believed. Consider- 

 able difficulty has, however, been ex- 

 perienced in giving a perfectly satisfac- 

 tory explanation of this remarkable 

 phenomenon. Mr. Leslie explains it in 

 accordance with his theory of pulsatory 

 undulations in the air, by supposing 

 that a portion of heat being abstracted 

 by the cold body from the air nearest 

 to it, the air so cooled is suddenly con- 

 tracted, which occasions the com- 

 mencement of a series of pulsations 

 accompanied with a discharge of heat 

 to the cold surface. Explanations in- 

 tended to agree with the usual theory of 

 radiant heat have been given by Pictet, 

 Prevost, and Martin. The prevailing 

 opinion is, that the phenomenon may 

 be explained by supposing that it arises 

 from radiation of heat in an opposite 

 direction ; the thermometer in this 

 case being the hotter body. Bodies ex- 

 posed to the air are all supposed to ra- 

 diate heat, at whatever temperatures 

 they may be ; and their temperatures are 

 raised when they receive more than they 

 radiate, and depressed when they radi- 

 ate more than they receive. Both the ice 

 and the thermometer radiate towards the 

 reflectors, before which they are placed ; 

 but the temperature of the ice being 

 lower than that of the thermometer, 

 it radiates less towards the thermome- 

 ter, than that does towards the ice; con- 



sequently, the temperature of the ther- 

 mometer is reduced. 



CHAPTER VII. 



Of Specific Caloric and of the different 

 Capacities of Bodies for Heat. 



THE first experiments in the investi- 

 gation of this subject were made by 

 Fahrenheit, at the desire of Boerhaave. 

 Equal quantities of the same fluid, such 

 as water, oil, or alcohol, being mingled 

 together at different temperatures, a 

 common temperature resulted : half the 

 excess of heat contained in oiie portion 

 being speedily imparted to the other, 

 until both became hot alike : but, when 

 fluids of different kinds were mingled 

 together, at different temperatures, very 

 different effects were observed. Water 

 and quicksilver, in equal bulks, added 

 to each other, the water being hottest, 

 gave a mixture, the heat of which was 

 higher than the medium between the 

 two : when the quicksilver \ras most 

 heated before mixture, the resulting tem- 

 perature was lower than the medium. 

 Three parts by volume of quicksilver 

 mixed with two of water, at different 

 temperatures, gave a mean temperature ; 

 as when equal volumes of water heated 

 to different degrees are mixed together. 



Boerhaave judged from these experi- 

 ments that heat is not distributed 

 through bodies in proportion to their 

 quantity of matter, since the effect pro- 

 duced by a quantity of mercury in 

 raising temperature is considerably less 

 than that of the same bulk of water, 

 although the former is thirteen times 

 heavier than the latter. 



Another inference was most inaccu- 

 rately drawn, by Boerhaave, from these 

 experiments. Convinced that heat is 

 not distributed among different bodies 

 in proportion to the quantity of matter 

 in each, he concluded, in opposition to 

 the very experiments from which he 

 drew the conclusion, that heat is distri- 

 buted in proportion to the space occu- 

 pied by each body ; and the same 

 opinion was adopted, without examina- 

 tion, by Muschenbroeck. 



The labours of Black, Wilcke, Irvine, 

 Crawford, Lavoisier, Gadolin, and other 

 philosophers, were applied to this diffi- 

 cult department of science, and the re- 

 sult obtained tended to prove that equal 

 weights or volumes of different bodies 

 contain unequal quantities of caloric at 

 any given temperature, 



