March 4, 1892.] 



SCIENCE. 



•35 



molecular motion in the surrounding medium. And the 

 brilliant incandescence in Geissler, Crookes, and Tyndall 

 tubes from minute iocrements of energy are well known. 



This increase of temperature and radiation from small in- 

 crements of energy in highly tenuous matter seems to be 

 what we ought to expect from the phenomena of this force 

 or energy when it is in the form of molar motion. We then 

 measure it by the mass and velocity of the moving body; 

 that is, by its momentum, and this momentum is what is con- 

 vertible into heat when the movement is resisted. 



Increase in velocity compensates for decrease in mass, and 

 hence a small projectile, at high velocity, will do the same 

 work as a larger projectile at lower velocity ; and the mo- 

 mentum, in each case, can be converted into the same units 

 of heat. For obviously the same reason, the intense velocity 

 imparted to the gaseous products of an explosion of dyna- 

 mite enables this highly tenuous matter to do precisely the 

 same work on a bard rock, as a hammer of a million times 

 the mass, but moving with only one-millionth of the ve- 

 locity. 



But there is necessarily a limit to this substitution of ve- 

 locity for mass; and this limit is in the capacity of matter to 

 embody the energy; and when the force of energy is applied 

 to matter in the form of heat we ought to expect to Snd the 

 same limit. This application in the form of heat may be 

 made by conduction, when the whole energy imparted is 

 absorbed; or by radiation when only so much as is not re- 

 flected, is absorbed; but the resulting phenomena are the 

 same, whatever may be the process by which the absorption 

 is accomplished. 



The fact developed in spectrum analysis, that incandescent 

 matter absorbs the same rays of light which it emits, seems 

 to be another illustration of the law that the capacity of 

 matter to receive radiant energy is limited, and in this case 

 by its capacity to radiate the energy received. 



If the evolution of heat and elevation of temperature re- 

 sults from resisted molecular motion, it necessarily follows, 

 that a single molecule, moving in u neon fined space, what- 

 ever may be its velocity, would be at the absolute zero of 

 temperature. But this is mere speculation of no scientific 

 value, because we have no evidence that a molecule can be- 

 come separated from other molecules, nor that it is possible 

 to place it where it could move without resistance. 



But there is another induction of practical importance in 

 sustaining the assumption that we have just made. If the 

 efPect of heat imparted to matter by conduction or radiation 

 is to set up the molecular motion evidenced by expansion, 

 and this work of molecular motion must be resisted before 

 radiation begins, it necessarily follows that the number of 

 molecules in the body receiving heat, and to which motion 

 can be imparted ; in other words, the density of tenuity of 

 the matter, must be an element, determining, in some meas- 

 ure, the capacity of the matter to absorb heat. 



This explains why the atmosphere decreases in tempera- 

 ture with increase of tenuity, upwards from the earth's sur- 

 face ;and why we can assume absolute zero in space entirely 

 unoccupied by ponderable matter, if there is any space thus 

 entirely unoccupied, notwithstanding the presence of poten- 

 tial or dynamic energy, because it is only in conjunction 

 with ponderable matter (resisted molar or molecular motion) 

 that dynamic energy develops elevation of temperature, and 

 the other phenomena of heat. 



It is obvious that force or energy in the form of molar 

 motion is being constantly converted by impact or friction 

 into the form of heat. Taking the earth as a whole, during 



the period of human observation, this constant conversion of 

 molar motion into heat has been compensated by a conver- 

 sion of heat into molar motion, so that the equilibrium be- 

 tween the two forms of this force or energy has been pre- 

 served in terrestrial nature, and there has been no loss of 

 motion nor increase of heat, since man began to observe 

 nature and keep a record of his observations. 



Resistance to movement, that is, to the work being done 

 by the force or energy in molar motion, is necessary to con- 

 vert the force or energy into the form of heat ; and it may 

 be that when this force of energy is applied to ponderable 

 matter in the form of heat, and its proper work as heat is 

 resisted, the surplus heat may be converted directly into molar 

 motion. 



It is certainly within the range of possibility, that, under 

 certain conditions, a body of ponderable matter may receive 

 increments of heat more rapidly than it can furnish work 

 for it in the molecular motion of expansion, or discharge it 

 by radiation or conduction; and, in such case, it seems in- 

 evitable that the body thus receiving more heat than it could 

 furnish work for or discharge, if free to move, would be put 

 in motion away from the source of heat, and that this mo- 

 tion would continue until a distance from the source of heat 

 was reached, at which the heat received was not greater than 

 could be employed in expansion or discharged in radiation 

 and conduction. 



Dr. Grove was inclined to the opinion that it was thus 

 possible to convert heat directly into molar motion. He 

 says, ''There are, indeed, some delicate experiments which 

 tend to prove that a repulsive action between separate masses 

 is produced by heat. Fresnel found that mobile bodies heated 

 in an exhausted receiver repelled each other to sensible dis- 

 tances; and Baden Powell found that the colored rings, 

 usually called Newton's rings, change their breadth and 

 position, when the glasses between which they appear are 

 heated, in a manner which showed that the glasses repelled 

 each other." ' 



But, however that may be, there is certainly a molar mo- 

 tion which always follows and evidences the molecular 

 motion of expansion. The law that action and reaction 

 must be equal and opposite, applies to molecular motion in a 

 closed vessel. It is the operation of this law which secures 

 uniform pressure in steam boilers, and other like devices for 

 using gas expansion for mechanical purposes; and thus con- 

 verts the molecular motion, evidenced by expansion, into 

 molar motion. Daniel S. Troy. 



(To be continued.) 



LETTEES TO THE EDITOR. 



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The editor will be glad to publish any queries consonant with the character 

 of the Journal. 



A Question in Physics. 



Can there be a crowding of the particles of a gas to a much 

 smaller compass without its being markedly heated ? Can a gas 

 expand without being cooled ? At first thought the answer would 

 seem to be an emphatic no in both cases ; but it would appear that 

 these conditions may exist sometimes. Science, Vol. XV., p. 387, 

 published the results obtained by direct determination of the 

 heating of air when compressed by a pump connected by a long 

 tube with the cylinder. A compression to ten inches above at- 

 mospheric pressure gave a heating of about 4° F., ignoring the 

 heat lost to the sides of the cylinder. The corresponding expan- 

 sion into the open air gave a cooling of about 4°, neglecting the 



> " Correlation and Conservation of Forces," p. 41. D. Appleton & Co. 1890. 



