496 



HEAT. 



earth, is changed into obscure heat which is 

 intercepted by the watery vapor of the atmos- 

 phere, and cannot therefore be radiated back 

 again into space. The atmospheric vapor is 

 therefore the means of maintaining the earth's 

 temperature, and if it were withdrawn from the 

 air, the loss of terrestrial heat would soon 

 render the earth uninhabitable. In all those 

 localities where the atmosphere is dry, the 

 nightly loss of radiant heat is so great, that 

 even in the burning desert of Sahara there is 

 nocturnal freezing. 



The aqueous vapor contained in the air ex- 

 ists mostly in its lower strata near the ground. 

 The upper portions of the atmosphere are com- 

 paratively dry. Hence, high mountains being 

 raised above the zone of watery vapor, are un- 

 protected, and their heat consequently streams 

 away into space with such rapidity that the 

 temperature sinks to a low degree. As the 

 winds dash against the frigid mountain peaks, 

 their moisture is rapidly condensed and frozen 

 into snow hence the everlasting snow of these 

 lofty land summits. In these circumstances, 

 where the snow falls incessantly in large quan- 

 tities, it is condensed into ice, and slowly creeps 

 down the valleys in the form of vast rivers of 

 ice known as glaciers. We thus see how the 

 relations of radiant heat to aqueous vapor af- 

 ford an explanation of the magnificent phenom- 

 ena of snow peaks and glacial action. The 

 ultimate cause of all these effects is of course 

 that solar heat which originally changed the 

 water into the vaporous form. The heat thus 

 absorbed must again escape in condensation, 

 while the grand function of the mountains ap- 

 pears as that of condensers. Each fragment 

 of glacial ice is to be regarded as the product 

 of the heat spent in first evaporating its water, 

 and in this point of view the glaciers represent 

 an amount of heat equal to .five times their 

 weight of melted cast iron. In connection 

 with these important discoveries of the opacity 

 of gases to radiant heat, Prof. T. Sterry Hunt 

 has called attention to the efiect which a large 

 proportion of carbonic acid in the earth's an- 

 cient atmosphere must have had in preserving 

 the high temperature of the earth. . 



The consummate series of investigations by 

 which these results were reached, is admirably 

 described by Dr. Tyndall, in his late work on 

 heat, in which the new views of the nature of 

 heat itself are applied with great skill and in- 

 genuity to many of the phenomena of nature. 



The history of the dynamical theory of heat 

 is deeply interesting, as throwing a striking 

 light on that action of the human mind which 

 leads to great discoveries of the laws of nature. 

 It illustrates, in a remarkable manner, that 

 great truths are growths of time, and that dis- 

 coveries oftener belong to epochs than to in- 

 dividuals. As far back as the time of Bacon, 

 we find statements concerning heat which con- 

 tradicted the common view, and which are sus- 

 ceptible of easy interpretation, in harmony with 

 the recently established views. In the twen- 



tieth aphorism of the second book of the No- 

 vum Organon, its illustrious author remarks: 

 " Now from this our first vintage, it follows 

 that the form, or true definition of heat (heat, 

 that is in relation to the universe, not simply 

 in relation to man), is in a few words as fol 

 lows : Heat is a motion, expansive, restrain- 

 ed, and acting in its strife upon the smaller 

 particles of bodies, but the expansion is thus 

 modified : while it expands all ways, it has at 

 the same time an inclination upward. And the 

 struggle in the particles is modified also ; it is 

 not sluggish, but hurried, and with violence." 

 Again, the philosopher Locke remarks: " Heat 

 is a very brisk agitation of the insensible parts 

 of an object, which produces in us that sensa- 

 tion from which we denominate the object, 

 but so that what in our sensations is heat, in 

 the object is nothing but motion." But the first 

 experimental step in this direction of thought, 

 and perhaps the grandest step taken by any 

 single mind, was made by an American, Benja- 

 min Thompson, afterward known as Count 

 Eumford. He went to Europe in the time of 

 the revolution, and devoting himself to scienti- 

 fic investigations, became the founder of the 

 Koyal Institution of England. He exploded the 

 notion of caloric, demonstrated experimentally 

 the conversion of mechanical force into heat, 

 and arrived at quantitative results, which, con- 

 sidering the roughness of his experiments are 

 remarkably near the established facts. He re- 

 volved a brass cannon against a steel borer by 

 horse power, for two and a half hours, and gen- 

 erated heat enough to raise 18f Ibs. of water 

 from 60 to 212. In his paper read before the 

 Royal Society, in 1798, he observes: "From 

 the results of these computations, it appears 

 that the quantity of heat produced equally in a 

 continuous stream, if I may use the expression, 

 by the friction of the blunt steel bar against 

 the bottom of the hollow metallic cylinder, was 

 greater than that produced in the combustion 

 of nine wax candles, each of an inch in diam- 

 eter, all burning together with clear bright, 

 flames." Eumford explicitly announced tho 

 view now held of the nature of heat and wrote 

 as follows, the italics being his own: "What; 

 is heat? Is there any such thing as an igneous 

 fluid? Is there anything that with propriety 

 can be called caloric ? We have seen that u 

 very considerable quantity of heat may be ex- 

 cited by the friction of two metallic surface's, 

 and given off in a constant stream or flux in all 

 directions. Without interruption or intermis- 

 sion, and without any signs of diminution or 

 exhaustion. In reasoning upon this circum- 

 stance, we must not forget that most remark- 

 able circumstance, that the source of the heat 

 generated by friction in these experiments, ap- 

 peared to be inexhaustible. It is hardly neces- 

 sary to add that anything, which any insulated 

 body or system of bodies can continue to fur- 

 nish without limitation-, cannot possibly be i 

 material substance ; and it appears to mo to ba 

 extremely difficult, if not quite impossible, to 





