132 



SCIENCE. 



[Vol. XIX. No 474 



SCIENCE: 



r NEWSPAPER OF ALL THE ARTS AND SCIENCES. 



PUBLISHED BV 



N. D. C. HODGES, 



874 Broadway, New York. 



Subscriptions. — United States and Canada S3.50 a year. 



Great Britain and Europe 4.50 a year. 



Communications will be welcomed from any quarter. Abstracts of scientific 

 papers are solicited, and one hundred copies of the issue containing such will 

 be mailed the author on request in advance. Rejected manuscripts will be 

 returned to the authors only when the requisite amount of postage accom- 

 panies the manuscript. Whatever is intended for insertion must be authenti- 

 cated by the name and address of the writer; not necessarily for publication, 

 but as a guaranty of good faith. We do not hold ourselves responsible for 

 any view or opinions expressed in the communications of our correspondents 



Attention is called to the "Wants" column. All are invited to use it in 

 soliciting information or seeking new positions. The name and address of 

 applicants should be given in full, so that answers will go direct to them. The 

 " Exchange " column is likewise open. 



For Advertising Rates apply to Henry F. Taylor, 47 Lafayette Place, New 

 Fork. 



MOTION AND HEAT. 



The term " Mechanical Equivalent of Heat " does not pie- 

 .sent a perfectly accurate concept of the determinations of Dr. 

 Joule and others. The great work actually done was the 

 determination of the " Heat Equivalent of Molar Motion." 



"The Mechanical Equivalent of Molar Motion" is the 

 amount of mechanical work that it will do; and when the 

 whole energy embodied in a given molar motion is coq- 

 verted into heat, the units of heat thus developed may again be 

 converted into molar motion capable of doing the same work. 

 Hence the term "Mechanical Equivalent of Heat" is accu- 

 rate enough for purposes of calculation. 



But the true equation is that molar motion is equivalent 

 to so much mechanical work; molar motion may be con- 

 verted into heat capable of the same amount of mechanical 

 work that the molar motion could do before its conversion 

 into heat, and therefore we have the "Mechanical Equiva- 

 lent of Heat." This use of the consequence, that is, the me- 

 chanical work which molar motion can do, for the motion 

 itself, tends to obscure the concept of the real relation be- 

 tween heat and molar motion. 



The primal work of the energy, or force, which constitutes 

 molar motion is to transfer a mass from one place, or part 

 of space, to another, and so long as this work is continued 

 and unresisted, no heat is developed. A body moving 

 through space entirely unresisted, whatever may be its mass 

 or velocity, develops no heat. It is only when the move- 

 ment is resisted by impact or friction of some kind that the 

 energy of motion assumes the form of heat; and it is only 

 when thus resisted that this energy of motion can do me- 

 chanical work. To the extent that the energy embodied in 

 resisted molar motion is expended in mechanical work it 

 cannot be converted into heat. 



Mechanical work consists in counteracting some other 

 force, generally gravitation or cohesion. The force or en- 

 ergy embodied in a ball thrown upwards from the earth's 

 surface develops no heat except such as may result from the 

 friction o*' the air; and if at its precise point of highest ele- 

 vation it lodges OQ the top of a house or some other support, 

 none of the energy is thereby converted into heat. The ball 

 has acquired what Mr. Balfour Stewart calls energy of posi- 

 tion; and when this potential energy again becomes dynamic 

 by the ball's falling to the earth, no heat isdeveloped except 



by the friction of the atmosphere, until the ball strikes the 

 surface of the earth. If the phenomena occurred in vacuum 

 neither the energy of motion in the ball, while doing the 

 work of lifting the ball to its highest elevation, thus coun- 

 teracting gravity, nor the potential energy rendered dynamic 

 by its fall, would develop any heat whatever until its impact 

 against the earth's surface. Here, according to the law of 

 conservation of energy, it would do work or develop heat 

 equivalent to that expended in its upward proiection. 



But to the extent that the energy of the impact itself does 

 mechanical work, that is, counteracts cohesion in the work 

 of molar deformation, it develops no heat. If an egg and a 

 metal ball of the same shape, size, and weight are dropped 

 from the same height on a hard pavement, the heat devel- 

 oped by the two impacts cannot be the same if the egg is 

 smashed. If the heat developed by the impact of the metal 

 ball is X, that developed by the impact of the egg must be 

 X minus the kinetic energy required to smash the egg. 



One of the occupations of my boyhood was to.attend a mill 

 for grinding corn, and one of the first things learned in that 

 business was that if the moving stone was properly balanced 

 and a sufficiency of corn supplied, the meal came out very 

 little heated ; but if the stones came into contact from lack 

 of having corn to grind or from want of proper adjustmentor 

 levelling of the moving stone, heat was developed rapidly. 



It is for this reason that hard substances like flint and 

 steel more readily develop by friction the heat necessary for 

 combustion than softer substances; the energy of motion in 

 the friction of softer substances is expended to a greater or 

 less extent in molar deformation, and it is only the residue 

 not thus expended that is available for conversion into heat. 



This principle is constantly applied in practical mechanics 

 to develop heat from friction when it is required, and to pre- 

 vent its development when not wanted. Except for igniting 

 combustibles, heat from friction is not generally wanted for 

 practical use; but Dr. Mayer mentions an instance in which 

 a manufactory used a surplus of water power to revolve two 

 large iron disks against each other to develop heat by fric- 

 tion to warm the establishment. The very general object in 

 mechanical work is to prevent the conversion of the energy 

 of motion into heat by friction, and this is done both by di- 

 minishing the frictional resistance, and also by the use of 

 solid lubricants whose molar deformation will furnish work 

 for the energy unavoidably lost in friction, and thus prevent 

 the development of heat and the local injury from the energy 

 in that form. 



Hence it was that Dr. Joule and others, in making the de- 

 terminations of the so-called "Mechanical Equivalent of 

 Heat," made use of substances in which there was no work 

 or very little work, of molar deformation for the energy, the 

 heat equivalent of which was measured. 



It seems, therefore, that two propositions may be stated: — 



First, that molar energy, that is, the kinetic energy of a 

 moving mass without friction, develops no heat while doing- 

 its primal work of transferring the mass from one place, or 

 part of space, to another. 



Second, that when the movement of the mass is resisted, 

 the lieat developed is the equivalent of only so much of its 

 energy as is not expended in molar deformation or other 

 mechanical work. 



There is obviously another cause which may prevent the 

 kinetic energy of molar motion from development into heat, 

 and that is its conversion into the molecular motion of ex- 

 pansion. When expansion occurs, there is necessarily an 

 enlargement of the intermoleeular spaces or of the molecules 



