March 4, 1892.] 



SCIENCE. 



133 



themselves, or a movement of the molecules ; and while we 

 have as yet uo such demonstration as is possible in molar 

 phenomena, we can assert, without fear of scientiflc denial, 

 that the phenomenon of expansion is a manifestation of mo- 

 lecular motion. 



It is usual to regard expansion as the work of heat, and it 

 is undoubtedly the work of the same energy which is em- 

 bodied in molar motion, and which causes an elevation of 

 temperature; which passes from one body to another by con- 

 duction, and from one body to another and from bodies into 

 space by radiation. 



But this energy, while doing the work of molecular mo- 

 tion in expansion, develops no more heat than while it is 

 moving an unimpeded mass through space. The impact of 

 moving masses free to expand from the instant of impact 

 could develop no perceptible heat; that is, only so much of 

 the enei'gy as was not expended in the work of molecular 

 motion of expansion would be available for the development 

 of heat. If the bodies brought into impact were liquid am- 

 monia, and this was set free in the atmosphere by the im- 

 pact, not only the entire energy of the impact (unless the 

 molar motion was almost beyond calculable velocity) would 

 be expended in the work of expansion, but energy in the 

 form of heat would be withdrawn from surrounding bodies 

 to finish the work. 



It is resistance to the molecular motion of expansion that 

 develops heat when expansion occurs as the primal work, 

 that is, which causes an elevation of temperature, and con- 

 verts the energy or force into the other well-known phenom- 

 ena of heat. 



This resistance may be from cohesion in the matter to 

 which the energy or force is imparted, from chemical aflBnity, 

 from the walls of a containing vessel or other environment, 

 or from a piston or other compression. In every case the 

 development of heat, that is, the elevation of temperature, 

 and the other phenomena indicating the conversion of the 

 force or energy into the form of heat, is determined by the 

 intensity of the molecular motion set up by the force or en- 

 ergy imparted to the body, and the resistance to it. 



Hence, in the experiments to determine the so-called 

 "Mechanical Equivalent of Heat" where expansion was 

 used, means were provided for its perfect resistance. Here 

 again the term does not express an accurate concept of the 

 determination actually made; it was really the "Heat 

 Equivalent of Molecular Motion." As in the other case, the 

 expression is accurate enough for purposes of calculation, 

 because the mechanical equivalent of molecular motion, 

 that is, the mechanical work it will do, is the same as the 

 mechanical equivalent of the heat developed by its perfect 

 resistance. 



It is not the motion in either case that is converted into 

 heat, but it is the force or energy causing the motion which 

 ceases to move the mass or molecules and causes an elevation 

 of temperature and the other phenomena of heat. 



It seems, therefore, that we can state two other proposi- 

 tions, namely : 



Third, that so much of molar motion as is converted into 

 molecular motion by impact or friction cannot be directly 

 converted into heat; and, 



Eourth, that the molecular motion set up by molar im- 

 pact, friction, or otherwise, and manifested by expansion, 

 can be converted into heat only by resistance to expansion. 



This force, or energy, is dynamic when causing motion or 

 when causing elevation of temperature and the other phenom- 

 ena of heat; but it becomes potential, or " energy of position," 



when a ball is thrown up and lodged on the roof of a house, or 

 when radiant and dynamic lieat becomes the latent heat of liq- 

 uefaction and evaporation, or when the dynamic radiation 

 from the sun is stored up in the molecular structure of the 

 hydro-carbons of vegetable and animal organisms by chemical 

 affinity and the vital forces; and it becomes partly potential 

 when heat is absorbed. 



This force, or energy, is directly subject to observation 

 only when dynamic; it apparently disappears when a ball 

 thrown up lodges on the roof of a house, or when heat be- 

 comes latent in liquefaction and evaporation, and when heat 

 and light are stored up in the molecular structure of vegeta- 

 ble organisms. But we know that by appropriate means it 

 can be rendered again dynamic, with its full integrity and 

 with the qualities it possessed before its imprisonment, in- 

 cluding the equivalence of its different forms. It becomes 

 dynamic in the form in which it was rendered potential; in 

 the ball loosed from its perch the energy becomes dynamic 

 as molar motion; in liquids and gases subjected to pressure 

 the latent heat of liquefaction and evaporation becomes 

 again dynamic as heat; and in the combustion of vegetable 

 organisms the sun's energy becomes again dynamic substan- 

 tially as it was locked up. 



Light is undoubtedly a division of the heat form of this 

 force, or energy. It is rendered potential in vegetable or- 

 ganisms, and becomes dynamic as heat, not as light, when 

 the com^Dustion of the organism occurs slowly and at a low 

 temperature. It not only results from intense heat, but 

 Professor Tyndall has demonstrated that heat rays, after 

 they leave the body which sends them forth, may be concen- 

 trated into light rays. It will therefore be sufficiently accu- 

 rate for our present purpose to consider both heat and light 

 as together constituting a single form of this force, or en- 

 ergy. 



If expansion is resisted by cohesion, chemical affinity, 

 mechanical pressure, or otherwise, the temperature of the 

 body rises in proportion to the increments of the force, or 

 energy, received; radiation increases with rise of tempera- 

 ture, and if the resistance is sufficient, incandescence and 

 the more intense radiation in the form of light, begin. 



It may be impossible from lack of power in any machine 

 which man can construct to compel by compression of ex- 

 panded matter incandescent radiation. But when heat be- 

 comes radiant as it does from compression, it is only a ques- 

 tion of intensity whether the matter radiating heat will be- 

 come red hot and radiate light also. 



In the combustion of hydrocarbons it is evidently the re- 

 sistance to expansion which causes heat radiation, and as 

 this resistance becomes more intense, light radiation also. 

 In the vegetable or animal organisms which constitute the 

 hydrocarbons a new molecular*structure has been built up, 

 in which force, or energy, coming dynamic from the sun 

 has been stored up and rendered as completely potential as 

 the energy of a ball lodged on the roof of a house, or as dy- 

 namic heat when it becomes the latent heat of liquefaction 

 or evaporation. This force, or energy, thus stored up by 

 chemical and vital action in the new molecular structure and 

 rendered potential, is set free and again rendered dynamic by 

 the chemical reaction of combustion, and the material ele- 

 ments return substantially to the condition in which they 

 were before. 



The force, or energy, thus set free by the chemical reaction 

 at once begins the work of dynamic energy; and if the mat- 

 ter in which the reaction occurs is free to expand, the energy 

 is expended in the molecular motion evidenced by expansion. 



