i34 



SCIENCE. 



[Vol. XIX. No. 474 



But if expansion is resisted by cohesion, chemical affinity, or 

 mechanical compression, there is an elevation of temperature 

 and the other phenomena of heat. 



As resistance to expansion increases, heat becomes more 

 intense; and when heat radiation is unable to carry off the 

 energy as rapidly as it is set free, the matter becomes incan- 

 descent, and the more intense form of light radiation be- 

 gins. 



The graphic description of ordinary combustion in Dr. 

 Josiah P. Cooke's "New Chemistry" leaves no doubt that 

 this is what actually occurs, and that " the light comes from 

 the incandescent solid particles,"' because they are more per- 

 sistent in resistance to the molecular motion evidenced by 

 expansion. The moment these particles are converted into 

 carbonic dioxide, and aqueous vapor, and thus become free 

 to expand, the matter ceases to be incandescent. 



If we could provide some means in ordinary combustion 

 for retaining the carbonic dioxide and aqueous vapor, with 

 the molecules concentrated as they are in the carbon parti- 

 cles, the matter would doubtless continue incandescent after 

 the reaction ; and undoubtedly the energy expended in the 

 expansion of the carbon dioxide and aqueous vapor, could 

 be converted into radiant heat by sufficient compression of 

 those gases. 



The phenomena of explosions demonstrate even more 

 clearly than ordinary combustion that the development of 

 heat results from resistance to molecular motion. Loose 

 gun-cotton exploded, will not develop heat sufficient to ignite 

 gTin-powder in contact with it; but if the gun-cotton is con- 

 fined, its combustion develops heat sufficient to ignite gun- 

 powder, and substances far more refractory. It is said that 

 the reason for this peculiar result of the explosion of loose 

 gun-cotton, is that there is not time to develop the heat. But 

 the true reason undoubtedly is that the molecular motion set 

 np is so intense, as compared to the resistance of the atmos- 

 phere, that the entire force or energy of the explosion is ex- 

 pended in that work, and there is little or no necessity for 

 elevation of temperature or radiation. 



In firing a gun, the energy developed by the explosion is 

 divided into three parts: that which by reason of resistance 

 to molecular motion causes elevation of temperature and 

 radiation in the barrel; that which imparts molar motion to 

 the projectile (which we know may also be converted into 

 heat) ; and. third, the i-esidue of molecular motion which is 

 dissipated in the atmosphere at the muzzle of the gun, and 

 neither develops heat in the barrel nor adds to the molar 

 motion of the projectile. 



If the foregoing inductions are sound, the heat developed 

 by an explosion is determined by the resistance to the mo- 

 lecular motion exerted by the force or energy set free and 

 rendered dynamic by the chemical reaction. This resistance 

 consists of cohesion and chemical affinity in the matter in 

 which the reaction occurs, and in the environment. If the 

 whole force or energy set free and rendered dynamic is d, 

 and the whole resistance is r, and x the units of heat devel- 



d 



oped, then a;= -. 



r 



This explains why the attempts made to determine the en- 

 ergy of explosives by the units of heat developed in their 

 explosion have resulted in unmitigated nonsense.' 



This has doubtless been a source of error in determining 

 the heat evolved or absorbed in chemical processes. The 



' The true measure of the energy of explosions must be the amount of 

 energy set free by the chemical reaction, and this Is determined by the num- 

 Iwr of molecules put In motion (quantity of matter, etc.) and their velocity. 



energy converted into heat by resistance to molecular motion, 

 and afterwards lost by radiation or conduction, is estimated 

 or otherwise taken into the account, but that which slips 

 away in the form of unresisted molecular motion is not 

 counted. 



"Although these values," says Dr. Cooke in his " Chemi- 

 cal Philosophy," "are undoubtedly as fundamental con- 

 stants of chemistry as the atomic weights, yet they have not 

 been as yet so fully confirmed or so thoroughly collated as 

 to enable us to present an entirely consistent system. Hence 

 the table here given [of heat evolved or absorbed in different 

 chemical actions] must be regarded as provisional, and as 

 serving only to illustrate the principles of the subject.' 



It is not necessary to the present inductions to determine 

 whether the molecular motion evidenced by expansion, and 

 which, when resisted, results in elevation of temperature and 

 other phenomena of heat, is molecular vibration as supposed 

 in the kinetic theory, or a rectilinear projection of the mole- 

 cules, as I have tried to prove.' All we need to know is that 

 this molecular motion, whatever may be its character or di- 

 rection, is work done, and, as irt the case of molar motion, 

 the energy embodied in it cannot be converted into heat ex- 

 cept by resistance. 



Elevation of temperature, whioh is the first phenomenon 

 of heat, seems to be a preparation for the flight of radiation, 

 the flight becoming more rapid or intense as the temperature 

 rises; but energy will not make the preparation nor begin the 

 flight from the matter in which it is embodied, unless its 

 work of molar or molecular motion is resisted or hindered. 



Whether heat absorbed by matter is energy rendered par- 

 tially potential by the partial counteraction of cohesion, or 

 whether it continues fully dynamic in the work of increased 

 molecular vibration as supposed in the kinetic theory, it is 

 not necessary for our present purpose to determine. We 

 know certainly that heat is absorbed by matter, and the 

 phenomena of the atmosphere demonstrate that the capacity 

 of matter to absorb heat diminishes by some as yet undeter- 

 mined ratio with increase of tenuity. This diminution of 

 capacity to absorb heat doubtless results from the smaller 

 number of molecules to which motion can be imparted; and 

 taken in connection with the induction that energy becomes 

 radiant as heat and light when molecular motion is resisted, 

 or hindered, it furnishes a very simple explanation of the in- 

 tense heat and brilliant incandescence which small incre- 

 ments of energy develop in highly exhausted tubes. 



The work of molecular motion being restricted by the 

 paucity of the molecules, the small increments of energy, 

 finding no sufficient work in moving them, elevation of tem- 

 perature and incandescence follow, for substantially the same 

 reason as in other cases where greater increments of energy 

 are applied. 



It seems to make no specific difference whether the incre- 

 ments of energy are imparted by the direct conduction or 

 radiation of heat, or by resistance to a current of electricity. 



Mr. Crookes, by concentrating increments of energy in a 

 highly exhausted tube on iridioplatinum alloy, one of the 

 most refractory metallic compounds, not only raised it to a 

 white heat, but actually melted it: while the same measura- 

 ble increments of energy applied to the same substance in 

 the atmosphere, or in some other medium not more tenuous, 

 would have caused hardly an appreciable elevation of tem- 

 perature. The energy, in such case, would be expended in 



2 " Chemical Philosophy," revised edition (1891), p. 174. 



5 " Molecular Motion in the Badiometer," etc. N. D. C. Hodges, New Tork, 



