Page:The American Cyclopædia (1879) Volume V.djvu/382

378 CORRELATION OF FORCES brisk agitation of the insensible parts of an object, which produces in us that sensation from which we denominate the object hot ; so that what in our sensations is heat, in the object is nothing but motion." These, how- ever, were only happy conjectures. It is to the American Count Rumford that the world is indebted for the first experiments designed to test the nature of heat, which broke down its old interpretation, and went far to estab- lish the modern theory. While engaged in the manufacture of ordnance at the arsenal in Mu- nich (179 6-' 8), Rumford's attention was arrest- ed by the large amount of heat resulting from friction in boring cannon, for which he could not account on the current hypothesis that it consisted of a material fluid. To satisfy him- self on this point, he made the following ex- periment. A steel borer 0-63 of an inch in diameter was pressed into the cavity of a brass cannon with a force of 10,000 Ibs., and made to revolve 32 times per minute. Heat was thus evolved in 2 hours sufficient to raise 18| Ibs. of water from 60 to the boiling point. Whence came this large amount of heat? The old view assumed that caloric was latent in the metal, and was set free by the condensation of friction, as a piece of metal is heated by con- densation in being hammered. But upon ex- amining the chips Rumford found that their " capacity for caloric " was the same as that of the metal before the experiment ; and that so large a quantity of heat could have been latent in a few grains of brass seemed impos- sible. Rumford therefore concluded that the heat was caused by friction, and was in the ratio of the power expended, .and therefore inferred it to be a motion communicated to the heated body. In his paper describing the ex- periment he said : " In reasoning upon this subject we must not forget that most remark- able circumstance, that the source of the heat generated by friction in these experiments ap- peared evidently to be inexhaustible. It is hard- ly necessary to add that anything which any insulated body or system of bodies can continue to furnish without limitation, cannot possibly be a material substance -, and it appears to me to be extremely difficult if not quite impos- sible to form any distinct idea of anything ca- pable of being excited and communicated in these experiments except it be motion," In view of these results Rumford asks : "Is there any such thing as an igneous fluid ? Is there anything that with propriety can be called caloric? " In 1799 Sir Humphry Davy melt- ed ice by rubbing two pieces of it together in a machine below the freezing point of water, which strongly confirmed the results of Rum- ford. The deathblow was thus given to the materialistic hypothesis of heat, and the idea gradually made its way in the minds of scien- tific thinkers that in all cases of friction or per- cussion the thermal effect is due to an arrest of mechanical motion and an increase of mo- lecular motion, the former being converted into the latter. When the idea became familiar that mechanical force is changed into heat, that is, that molar motion is transformed into molecular motion, it naturally led to the re- verse view, that is, the reconversion of heat into mechanical force. A familiar example of this is the steam engine, in which heat pro- duces molecular expansion in water, which is then transferred to the piston and produces mechanical effects. But if there be this recip- rocal relation between mechanical force and heat, the unavoidable question arises as to the quantitative relations of the phenomena. How much mechanical force is equivalent to a given amount of heat, and vice versa ? Carnot, a French engineer, undertook in 1824 to formu- late this relation in the case of the steam en- gine, by establishing the law that the greatest possible work of a heat engine is related to the amount of change of temperature under- gone during the action of such engine by the enclosed elastic body. This, however, was a fundamental question of great importance, re- quiring the most careful experimental deter- mination, and it was entered upon by several scientists of different countries about the same time. Dr. J. P. Joule of Manchester, England, has the honor of first establishing experimen- tally what is called the "mechanical equivalent of heat" His mode of proceeding was to agi- tate different liquids, such as water, mercury, and oil, in suitable vessels, by paddles driven by falling weights. The friction produced heat, which raising the temperature of the liquids was carefully measured and its amount taken as the equivalent of the mechanical force expended. He also rubbed cast-iron disks against each other, carefully determining the force employed and the heat produced. As a result of a large range of trials with liquids and solids, he found that the same expenditure of power gave the same absolute amount of heat, whatever the substance used for pro- ducing friction. The average result of a long course of experiments was that 772 Ibs. falling through one foot (that is, 772 foot pounds) produced sufficient heat tp raise one pound of water 1 F. and conversely, one pound of water falling through one degree of tem- perature gives out heat enough to raise by ex- pansion 772 Ibs. one foot high. This is known as the " thermodynamic unit," or "Joule's equivalent. " The quantitative investigation' of the relations of forces now proceeded rapidly, and Joule's result was confirmed in other ways. It was found that an electric current which by resistance in passing through an imperfect con- ductor produces sufficient heat to raise one pound of water 1, sets free an amount of hydrogen which when burned raises exact- ly one pound of water 1 ; while the same amount of electricity will produce a magnetic force by which 772 Ibs. may be raised one foot high. Thus electricity, magnetism, and chem- ical force were brought into numerical corre- lation with heat and mechanical power. Joule's