HEAT. 



heating water in a tube similar to that 

 in the last experiment but one, the 

 water having some particles diffused 

 in it of amber or other light substance 

 not soluble in water. 



From these and similar experiments, 

 it appears that the reason why heat is 

 so slowly diffused through liquids in a 

 downward direction, is, that the heated 

 particles being expanded are rendered 

 specifically lighter than those imme- 

 diately above them, and therefore im- 

 mediately ascend, without having had 

 time to impart heat to the particles below. 



Count Rumford also endeavoured to 

 prove that the gases are non-conduc- 

 tors of caloric : the truth of this con- 

 clusion, however, is not admitted, but 

 the experiments prove that whatever 

 obstructs the motion of the particles 

 of air, renders the propagation of heat 

 slower. 



The facility with which an air ther- 

 mometer shews changes of tempera- 

 ture, was adduced by Berthollet, to 

 shew that air must be a good conductof 

 of heat ; and he refers to the fact that 

 in air-balloons the gas has been found 

 to expand suddenly when the sun be- 

 came unclouded. In explaining this 

 effect, it appeared to him impossible that 

 the particles of the gas within the 

 balloon could be so quickly heated by 

 coming individually and successively 

 into contact with its sides. He infers, 

 therefore, that the heat must have been 

 conducted by the gas itself. He ac- 

 counts for the effect which, it is acknow- 

 ledged, confined air has in retarding the 

 communication of heat, by supposing it 

 to depend upon a degree of compression 

 by which its expansion is prevented, 

 a solution not very satisfactory. 



It has been ascertained by Mr. Leslie, 

 that bodies require different times to 

 cool in different gases. In hydrogen 

 the process of cooling goes on rapidly, 

 not so quick in atmospheric air, and 

 much slower in carbonic acid gas. 



In describing the effects produced by 

 expansion, the agency of air in distri- 

 buting heat has already been adverted 

 to. The expansion and consequent as- 

 cension of successive portions of air 

 from heated parts of the earth's surface, 

 occasions a current of air to flow from 

 the poles towards the equator, near 

 the surface of the earth ; a superior 

 current from the equator towards the 

 poles, modifying and regulating tempe- 

 rature over the globe. 



The agency of water is of equal im- 



portance. Cold air passing over the 

 water of the sea, from the polar regions, 

 is much warmed in its progress, by heat 

 derived from the water. Count Rum- 

 ford affirms, that one cubical foot of 

 water, in cooling one degree, gives out 

 a sufficient quantity of heat to raise 

 the temperature of a stratum of air 

 over it, forty-four times as thick as the 

 water, ten degrees. The Count enter- 

 tained an opinion that the water which 

 imparts its heat, and which descends, 

 in consequence of its increased specific 

 gravity, flows towards the equator, oc- 

 casioning a current at the surface, in 

 an opposite direction ; tending, like the 

 currents in the air, to moderate the ex- 

 tremes of temperature. 



It is impossible to consider these, and 

 similar silent and unobtrusive, but ex- 

 tensive and most useful operations, with- 

 out being deeply affected by a sense of 

 the wisdom and power by which they 

 were contrived and carried into effect. 



CHAPTER VI. 

 Radiation of Heat. 



WHEN heated bodies are exposed to 

 the air, they lose portions of their heat 

 by projection, in right lines into space, 

 from all parts of their surfaces. The 

 investigations by which this interesting 

 property of caloric has been made fully 

 known are of recent date, although it 

 was not entirely unknown at an earlier 

 period. In 1682 it was mentioned by 

 Marriotte, in the Memoirs of the Aca- 

 demy of Sciences of Paris; he pointed 

 out the fact, that the heat of a fire, 

 which is rendered sensible in the focus 

 of a burning mirror, ceases to be sensi- 

 ble when a glass is interposed. Having 

 found that substances may be inflamed 

 at a distance of twenty or twenty-four 

 feet, by burning charcoal placed between 

 two concave reflectors, Lambert, in 

 order to ascertain if any part of the 

 effect was occasioned by light, collect- 

 ed the light of a clear fire by a large 

 lens, but could scarcely discover any 

 heat in its focus. 



In the celebrated Treatise on Air 

 and Fire, by Scheele, similar experi- 

 ments are detailed, with important ad- 

 ditions ; the term Radiant Heat, or heat 

 flying off like light in rays, originated 

 with him, as did also the knowledge that 

 it passes through the air, without heat- 

 ing it ; and that its direction is not 

 changed by a current of air. He ob- 

 served that glass, which permits the 



