

i IK AT. I. 



ITS NATURE MATERIAL AND DTKAJCICAL THEORIES 

 PHYSK \i. 1C SOURCES OF BEAT. 



commencement of oar lessons oa tbo subject of Heat a 



- which wo cannot altogether overcome { that 



really U. It U, bowerer, BO 



I'.uiuli. the offooU it produces on our bodies, and on 



. : us, tii. a wo need not bo at all deterred 



; iii>iuin<'.s liy ..in- inability to Bolvo thU qucnt 



> are two theories on the rabjoct which hare been hold 



by ililTi'ivnt philosophers. According to the former of those, 



: oin a subtle imponderable substance called caloric, 



whi.-h MirrouuJs llu> ultiinnto atom* of all substances, and L* 



cui>u.l>' ' from ouo body to another. This is known 



as tin- theory, and was for a long timo almost 



il. According to it, no now boat could bo 

 called into existence, the amount in tho universe being constant, 



a tho only way of heating any substance was by trans- 

 ferring some of this caloric 

 from some other substance 

 i was charged with it. 



Many of tho ordinary 

 mt'iui could be very 

 easily explained upon this 

 theory, but tho production 

 of an almost unlimited 

 amount of heat by friction 

 could not bo accounted for, 

 xinco it was obvious that no 

 heat could be evolved in this 

 way that was not previously 

 stored up in tho substance. 

 An extensive series of ex- 

 periments on the question 

 was conducted by Count 

 llumfonl, and led to tho 

 rejection by many of the 

 material theory ; and a 

 celebrated experiment by 

 Sir H. Davy fully supported 

 these views. 



When ice becomes melted 

 a large amount of heat dis- 

 appears or becomes latent, 

 as will be fully explained 

 hereafter ; the water, there- 

 fore, according to the ma- 

 terial theory, must possess 

 a much larger amount of 

 caloric than the ice. Davy 

 accordingly took two lumps 

 of ice and rubbed them 



violently together, and in a short time found that a portion of 

 tho ice was liquefied by tho friction. Now, as in this instance 

 the water contained a greater amount of heat than the ice, it is 

 clear that there must have been an actual production of heat, 

 and not merely tho evolution of some which was previously 

 latent. 



These experiments, with many others which might be quoted, 

 tend strongly to disprove the material theory, and to support 

 the other or dynamical theory. According to this, heat is not 

 a material substance, but a motion of the ultimate particles of 

 which bodies are composed. In this way heat presents many 

 analogies to light and sound, for it is supposed that just as sound 

 is produced "by tho vibrations of the air, so heat results from 

 the rapid vibration of the particles of any body, or, according 

 to some, tho vibrations of an imponderable fluid by which those 

 particles are surrounded. In further support of this view, we 

 find that motion is frequently converted into heat, and con- 

 Tersely, that heat may bo changed into motion. 



When a ball is allowed to fall from a height, it acquires in 

 falling a considerable velocity. Let it now strike upon some 

 hard substance, and it will immediately be brought to rest ; its 

 motion will not, however, bo annihilated, as will bo seen if we 

 allow a ball to fall several timer!, and then carefully ascertain 

 its temperature. We shall then find that tho motion of the 

 ball has been changed into a motion of its ultimate particles, 



122 X.K. 



U manifested by the increased 



investigation -hows thai this increase U 



> ; 



Om 



\ .:.. | , 



the height (ram which the ball has fallen 



We cannot, in toe spaoe of these lissnas. fully inquire iato 

 this matter ; the student is. however, rsmrnmended to ummt 

 carefully the treatise of Profeseor TyadaU, entitled * Heat a 

 Mode of Motion." which will give him a fall iasigfct into the 

 results of modern research on this subject. 



The chief physical source of the heat which we enjoy fe Ik* 

 son. which, although situated at such an immense distance few 

 as, nerertheloM warms the earth by ite my*. Of the soaree of 

 the solar heat nothing is known, although maoy very okret 

 hypotheses, as well as many very foolish ones, have been 

 started. So great is the amount of solar boat received by the 

 earth that it has been calculated that it would be sofieient to 

 molt in a year a layer of ioe surrounding the globe to atttekoeM 

 of 38 yards. In addition to this external source of beat, we 

 hare an internal source known as terrestrial boat. 

 As we dig down into tho substanoe of the earth, wo find tho 



temperature diminishes a 

 little in summer till we 

 attain a depth of about M 

 yards. At this depth it 

 remains orairtant all the 



..-.:. .:.!.-.. -:... 



and the winter frost being 

 alike unable to penetrate; 

 the temperature of this 

 stratum is about &8. If 

 now wo sink still deeper, the 

 temperature is found to in- 

 crease at the rate of one 

 degree for every 60 or 70 

 feet, and this increase con- 

 tinues nearly uniform, what* 

 ever depth we attain. It is 

 evident, therefore, that ata 

 depth of a few miles the 

 temperature must be TOT/ 

 high ; it is sometimes state i 

 that at a depth of eight or 

 ten miles this beat would be 

 so great that nothing could 

 resist it, and even the 

 hardest rock would be fused. 

 The melting-point of any 

 body becomes, however, 

 higher as the pressure on it 

 is increased, and thus it is 

 probable that the thinkmss 

 of the crust of the earth is 

 far greater than this. Many 

 astronomical observations 

 seem likewise to point to the same conclusion. 



The air resting on tho earth becomes warmed by contact with 

 it, and by its radiation, so that as we ascend above the earth'* 

 surface the temperature gradually diminishes at the rate of 

 about ono degree for every 300 feet of elevation. 



Though wo ore mainly dependent on these physical sources 

 for maintaining our temperature, there are i~hMWl and 

 chemical sources of heat which are of great importance to us, 

 and to which we must accordingly direct our attention. 



The first of these sources of heat is friction, and the simplest 

 experiment that can be tried, as illustrating the production of 

 heat in this way, is to rub a metal button or the blade of a knife 

 rapidly to and fro on a piece of wood. It will soon become so 

 hot that it cannot be touched with any degree of comfort, and 

 a piece of phosphorus may easily be inflamed by contact with it. 

 The Indians are aware of the fact that heat may thus be evolved, 

 and often obtain a fire by rubbing one piece of wood violently 

 backwards and forwards upon another. A little loose powder 

 or dust soon accumulates in a groove on the lower piece, and 

 the heat becomes sufficiently intense to set light to this. 



The experiments of Count Bamford on this subject moot be 

 described here, as they were carefully arranged and conducted, 

 and are very frequently referred to. He was engaged in super- 

 intending the boring of cannon at Munich, and in the course of 

 this was struck with the groat amount of heat evolved dnanc 



