June 13, 1872] 



NATURE 



123 



think of it, can either produce energy or do work, it is 

 difficult to conceive; has this "any connection with 

 sound philosophy " ? and does not the reviewer here him- 

 self " forget that each thing is itself, and not something 

 else " (p/22, 1. 89). 



The writer would commend to the serious attention of 

 his brother-physicists the above definitions ; he would also 

 submitthe following definitions of sound, light, and heat, the 

 former of which has,however,been elsewhere declared to be 

 incapable of definition,* as well as by the reviewer (p. 23). 

 Sound may be defined to be the impression produced by 

 certain vibratory movements of matter upon appropriate 

 sensuous organs, including both the receptive and per- 

 ceptive apparatus. Whether the tympanum be incapable 

 of receiving sonorous vibrations, or whether it vibrate 

 sympathetically while the structures of the internal ear 

 are incapable of appreciating its vibrations, there can be 

 no sound. 



And why may not the same definition apply to light and 

 heat .' It is, in fact, far from improbable that the percep- 

 tions of light and heat may result from the impressions 

 produced by the same identical vibrations on different re- 

 ceptive organs ; that of light on the eye and its nervous 

 appendages, that of heat on the organs of common sensa- 

 tion. In common parlance, the terms sound, light, heat, will 

 doubtless continue to be applied indiscriminately to the 

 vibratory motion producing, and to the impressions pro- 

 duced ; and to this there can be no objection, provided no 

 advantage be taken of the misnomer to found thereon an 

 assumption of the identity of the pro.ximate cause and the 

 resulting effect. 



The reviewer has sought to kill two birds with one 

 stone, and has made a vigorous onslaught against the 

 conservation of energy in general, and the writer in par- 

 ticular, regarding the theoiy of latent heat ; but it may 

 reasonably be questioned how far " sound philosophy " is 

 shown in attempting to convict an author of admitting an 

 insuperable difficulty in the adoption of a given piinciple 

 by quoting his statement of the difticulty, and coolly sup- 

 pressing his immediately subsequent explanation of it. 

 He thus quotes the writert : — " Latent heat has ever been 

 held up as the great stumbling-block of the dynamic 

 theory, because it is impossible to conceive motion to be 

 reduced to a state of quiescence, but remaining still ready 

 to start again into action." 



But instead of continuing the quotation thus : — " This, 

 however, is merely a confusion of ideas, the fact being 

 that when any substance passes from the soUd to the 

 liquid, or from the liquid to the gaseous form, a certain 

 portion of the impressed heat-force is continuously occu- 

 pied in overcoming molecular attraction, and thereby 

 effecting the change of form ; and this heat cannot be im- 

 parted to other bodies so long as it is occupied in main- 

 taining that change," he ventures to state : — " In this we 

 quite agree, and hence we think it unnecessary to give 

 Mr. Brooke's reasons for believing a doctrine which he 

 allows to be inconceivable " ! ! ! Whether " sound 

 philosophy" or not, is this common literary honesty? 

 What IVIr. Brooke allows to be inconceivable is obviously 

 not the doctrine itself, but (the conclusion drawn from 

 fallacious arguments adduced in opposition to it ; for to 

 assign reasons for believing what one allows to be incon- 

 ceivable would be nothing less than pure and simple 

 evidence of mental alienation. 



The fact is that the term " latent heat " is an unfortu- 

 nate one, and has mystified the reviewer, as well as many 

 others. It ought long ago to have been consigned to the 

 limbo of discarded hypotheses, together with the material 

 or corpuscular theory of heat from which it arose. If 

 heat consisted of material particles, it might be supposed 

 to become latent among the molecules of grosser matter, 



* "A logical definition of sound is impossible." Dr. M'Cann on "Force 

 and its Manifestations," a paper lately read before the Victoria Institute, 

 t Elements of Natural Philosophy (p. 786). 



just as a handful of shot, if dropped into a bo.x full of 

 marbles, would lie hid amongst them, and be lost to sense, 

 and would so remain until shaken out again ; but mere 

 vibratory motion cannot be theoretically dealt with in this 

 fashion. 



A much better term would be engaged or occiipieel heat, 

 for the so-called latent heat is wholly engaged or occupied 

 in maintaining the change — first from the solid to the fluid 

 state, and secondly from the fluid to the gaseous. The 

 facts are very plain ; a pound of water at the temperature 

 of 0° C, or the freezing point, mi.xed with a pound of water 

 at 79° yields two pounds at the mean temperature of 

 39'5" ; but a pound of ice or dry snow at the tempera- 

 ture of 0° mixed a pound of water at 79' yields two 

 pounds of water at 0°, because the 79° of sensible heat 

 in the water are now employed or occupied in maintaining 

 such an amount of vibratory motion in the molecules of 

 the ice, that they are no longer able to obey that polar 

 attraction by which they were previously aggregated to- 

 gether in given directions so as to form crystals (for 

 though not so evident in ice, the crystalline character of 

 snow is notorious), and the heat-energy, being thus occu- 

 pied in doing work, is incapable of doing any other work, 

 as for example on the organs of sensation, at the same 

 time ; on the principle that you cannot "eat your cake, and 

 have it too." The same reasoning applies to the change 

 from the fluid to the gaseous state ; but in this case a 

 much larger amount of thermic energy is employed in so 

 far removing the molecules from the sphere of each other's 

 attractions that the balance of their mutual forces is re- 

 pulsive, and so long as that repulsion is maintained, the 

 dry steam manifests all the properties common to the 

 fixed gases. " Latent " heat, then, when properly under- 

 stood, ceases to be a " stumbling-block to the dynamic 

 theory of heat." 



One finds oneself occasionally brought by circumstances 

 into an unwelcome generalisation. Tnus the reviewer, 

 speaking of the supporters of " conservation " in the 

 lump, says (p. 21) " they take it for granted that force is 

 motion and nothing but motion." This the writer entirely 

 and absolutely denies. The reviewer, then immediately 

 preceding his observations on the writer's views, quotes 

 from Prof. Bain that " Inert matter in motion is force 

 under every manifestation." This is plainly an abuse of 

 language, in which the writer, as one of the " they," de- 

 clines to participate ; inertness and force are hardly con- 

 comitant ideas, and matter, whether in motion or at rest, 

 is assuredly not force. The term heat-potential adopted 

 by Mr. Rankine is admissible only in relation to heat as 

 previously defined ; the thermic energy can hardly be 

 termed potential while it is employed in doing work. 



The reviewer (p. 19) quotes, and objects to, the explana- 

 tion of latent heat ottered by Prof. Tail : that while sen- 

 sible heat is motion, latent heat is position. The writer 

 must acknowledge his inability to derive any definite idea 

 from this statement of Prof. Tait ; he cannot therefore 

 express either assent or dissent. 



The writer must plead guilty to having made use in the 

 treatise above referred to, in common with many others, of 

 a phrase which is not strictly accurate, viz., that sound, 

 light, heat, and electricity are modes of motion. It would 

 be more exact to state that they are so many forms of 

 energy, or effects due to matter affected by certain modes 

 of motion. 



It is rather surprising that the reviewer should have 

 ventured to dogmatise on the very slender knowledge 

 either of physical facts or hypotheses that he evidently 

 possesses. Thus he states (p. 33, note) regarding the in- 

 vestigations of Dr. Joule : — ■ 



" By means of machinery a weight of 772 lbs. is made 

 to turn a small paddle-wheel placed in one pound of water. 

 Dr. Joule found that the descent of the weight with a given 

 velocity through one foot raised the temperature of the 

 water exactly TF." Now this small sentence contains 



