40 



C H E M I S T R Y. 



Nitrous oxide 0.549 



Carbonic acid (). i^i 



Ammonia 1. 



Carbureted hydrogen l.SSS 



Olefiant gas 1 



Nitric acid 0.4JU 



Carbonic oxide 0.777 



Sulphureted hydrogen 0. 



Muriatic acid 0.424 



Aqueous vapour 1.166' 



Ether vapour 0.848 



Alcohol vapour 0.58G 



Dr Crawford supposed, that the specific heat of 

 bodies is permanent while they retain their state. 

 But Mr Dalton has lately endeavoured to prove, that 

 it increases with the temperature of all bodies. 



Dr Irvine ascertained, that the specific caloric al- 

 ways changes with the state of a body. When a so- 

 lid becomes a liquid, or a liquid an elastic fluid, the 

 specific caloric increases. When an elastic fluid be- 

 comes a liquid, or a liquid a solid, the specific heat 

 diminishes. 



The specific heat of bodies is increased by com- 

 bining them with oxygen. Thus, the specific heat 

 of metallic oxides is greater than that of metals, and 

 of acids than of their bases. 



2. Of the Absolute Quantity of Caloric in Bodies. 



As the same quantity of heat produces different 

 changes of temperature in different bodies, it is ob- 

 vious, that the thermometer cannot indicate the ab- 

 solute quantity of heat in bodies. Now, it becomes 

 a question of considerable importance to inquire, if 

 there be any method of ascertaining the absolute 

 quantity of heat in bodies. Supposing a body de- 

 prived of all heat, and a thermometer applied to it, 

 at what point would the thermometer stand ? 

 Absolute Dr Irvine is the philosopher who first attempted 



heat, how to solve this problem. His reasoning wa founded 

 ascertained. U p On t wo suppositions. 1. That the specific heat 

 was proportional to the absolute heat of bodies. 2. 

 That the heat emitted or absorbed by a body, when 

 it changes its state, is merely the consequence of the 

 change which has taken place in its specific heat. 

 Thus, when ice is converted into water, 140 of heat 

 are absorbed ; because the specific heat of water is 

 so much greater than that or ice, that 140 are ne- 

 cessary to maintain the temperature. The first of 

 these two suppositions gave him the ratio of the ab- 

 solute quantity of heat in bodies, and the second, 

 the difference between two absolute calorics. Thus, 

 if the specific heat of water be 10, and that of ice 

 9, then the absolute quantity of heat in water is, to 

 that in ice, as 10 to 9. Call the absolute heat of ice 

 x, then that of water is .r-f- 140, and we have a: :i + 

 140: :9: 10. Hence we get this equation, 10xr= 

 9 *+ 1260, which gives us J= 1260. Water at 32 

 of course contains 1400* of caloric. Dr Crawford, 

 from his experiments, stated the real zero at 1500 

 below 0; and Mr Dalton places it at 6000 below 

 0. 



Unfortunately, the truth of the two suppositions 

 upon which this ingenious reasoning is founded, can- 

 not be admitted. We have no proof that the specific 



heat of bodies is proportional to their absolute heat. Element! 

 The second supposition is at variance with the me- ' 

 chanical phenomena which present themselves when ^"_ cm " 

 substances change their stato, and would leave that 

 change itself unaccounted for. It cannot, therefore, 

 be admitted. Various other methods of ascertaining 

 the absolute heat of bodies have been proposed ; but, 

 as they are all unsatisfactory, it is not necessary to 

 detail them here. 



SECT. VI. Of the Sotircct of Caloric. 



The most important sources of heat are the five Of the 

 following, the sun, combustion, percussion, friction, sources of 

 and mixture. caloric. 



1. The Sun. 



The sun is an immense globe, the diameter of which The sun. 

 is 888,246 miles. It was long supposed to be in a 

 state of violent combustion. But the curious obser- 

 vations of Dr Herschel render it probable that this 

 notion is erroneous. From them it appears, that the 

 sun is an opaque globe, surrounded by an atmosphere 

 of great density and extent. In this atmosphere, 

 there float two regions of clouds. The lowermost of 

 the two is opake, and similar to the clouds which 

 form in our own atmosphere ; but the higher region 

 of clouds is luminous, and emits the immense quanti- 

 ty of light to which the splendour of the sun is ow- 

 ing. 



The sun emits three kinds of rays ; the calorific, Solar rays. 

 colorific, and deoxidizing. The first occasions heat, 

 the second colour, and the third separates oxygen from 

 various bodies. 



When the solar rays strike transparent bodiep, 

 they produce very little effect ; but opake bodies art- 

 heated by them. They pass through transparent 

 bodies ; but are retained, at least in part, by opake 

 bodies. The deeper the colour of the opake body, 

 the greater is the heat produced. Black bodies are 

 most heated, and white least, and the others in pro- 

 portion to the intensity of the colour. The temper- 

 ature produced in bodies by the direct action of the 

 sun's rays, seldom exceeds 120. But when the 

 heat is prevented from escaping, as, by enclosing a 

 thermometer within a glass vessel whose bottom is 

 cork, the temperature sometimes rises nearly to 240. 

 When the sun's rays are accumulated by means of 

 burning glasses, the most intense heat is produced 

 that it is possible to raise by any known method. 



2. Combustion. 



Few phenomena are more wonderful or interesting combus. 

 than combustion. When a stone or a brick is heated t ion. 

 it undergoes no change, and, when left to itself, it 

 soon cools again, and becomes as at first. But, when 

 combustible bodies are heated to a certain degree in 

 the open air, they suddenly become much hotter of 

 themselves, continue for a certain time intensely hot, 

 and send out a copious stream of light and heat. 

 When this ceases, the combustible has undergone a 

 most complete change, being converted into a sub- 

 stance possessed of very different properties, and no 

 longer capable of combustion. 



The first ingenious attempt to explain combustion 



