520 



NA TURE 



[SErXEMBER 24, 1908 



applicable to all bodies — solid, liquid, and gaseous. On 

 the " caloric " or " material " theory of heat, motive 

 power is obtained during the letting down or fall from a 

 higher to a lower level of a given quantity of heat. The 

 quanti'iV of heat does not alter in the process ; it is only 

 its relative level which alters. There is no reason, there- 

 fore, for mentally limiting the amount of mechanical energy 

 obtainable from any given quantity of caloric, just as there 

 is no reason for limiting the amount of mechanical energy 

 to be mentally derived from a given weight. Any desired 

 quantity of energy may be derived from a weight of, say, 

 one pound, if it only be allowed to fall far enough, 

 assuming gravity to be constant through the range. 



The investigation of the work to be derived from a 

 given quantity of heat at a given temf)erature is thus a 

 matter of experiment, which can be settled by measure- 

 ment of the properties of a few bodies. 



Reasoning, it is conceived, in this way, Thomson follows 

 up his absolute thermometric scale work with an investi- 

 gation entitled " Carnot's Theory of the Motive Power of 

 Heat," described in a Paper read in 1S49 before the Royal 

 Society of Edinburgh, in which he calculates from Reg- 

 nault's experiments on steam the power developed by a 

 Carnot reversible engine when using one centigrade heat 

 unit ; that is, the heat necessary to heat one pound of 

 water through 1° C. for temperatures from 1° to 231° C, 

 the temperature falling in the engine in each case to 0° C. 



In this Paper he asks himself two questions : (i) What 

 is the precise nature of the thermal agency by means of 

 which mechanical effect is to be produced without effects 

 of any other kind? and (2) How inay the amount of the 

 thermal agency necessary for performing a given quantity 

 of work be estimated? 



Using Regnault's values for the properties of steam, he 

 calculates the lines of compression and expansion without 

 heat loss, the lines of compression and expansion with 

 heat flow at the lowest temperature, and heat addition at 

 the highest temperature, and thus arrives at the work 

 area per heat unit let down. He tabulates these results, 

 and shows that what he calls Carnot's function diminishes 

 as temperature rises, using the ordinary centigrade scale. 

 On the caloric theory the methods are rigidly logical and 

 correct, but some inaccuracy is introduced by the necessity 

 of that theory for the discharge of the same amount of 

 heat at the third operation as is taken in on the first. 

 The Paper is of great interest, however, because it shows 

 clearly how fully the distinguished author realises the 

 necessity for re-examining the standard ideas of the nature 

 of heat. Two paragraphs make this very clear : — 



" 7. Since the time when Carnot thus expressed him- 

 self the necessity of a most careful" examination of the 

 entire experimental basis of the theorv of heat has become 

 more and more urgent. Especially all those assumptions 

 depending on the idea that heat is a substance, invariable 

 in quantity, not convertible into any other element, and 

 incapable of being generated by any physical agency ; in 

 fact, the acknowledged principles of latent heat would 

 require to be tested by a most searching investigation 

 before they ought to be admitted, as they usually have 

 been, by almost everyone who has been engaged on the 

 subject, whether in combining the results of experimental 

 research or in general theoretical investigations. 



" 8. The extremely important discoveries recently made 

 by Mr. Joule, of Manchester, that heat is evolved in every 

 part of a closed electric conductor moving in the neigh- 

 bourhood of a magnet, and that heat is generated by the 

 friction of fluids in motion, seem to overturn the opinion 

 commonly held that heat cannot be generated, but only 

 produced from a source where it has previouslv existed 

 either in a sensible or in a latent condition. In the pre- 

 sent state of science, however, no operation is known by 

 which heat can be absorbed into a body without either 

 elevating its temperature or becoming latent, and pro- 

 ducing some alteration in its phvsical condition ; and the 

 fundamental axiom adopted by Carnot mav be considered 

 as still the most probable basis for an investigation of 

 the motive power of heat, although this, and with it every 

 other branch of the theory of heat, may ultimately require 

 to be reconstructed upon another foundation when our 

 experimental d.ita are more complete. On this understand- 

 ing, and to avoid a repetition nf doubts, 1 shall refer to 



Carnot's fundamental principle, in all that follows, as if 

 its truth were thoroughly established." 



In these two paragraphs Thomson sums up the whole 

 situation in 1849, and promises further investigation and 

 further attempts to deduce the nature of the connection 

 between heat and work. 



Assume, then, the truth of the caloric theory of heat, 

 as Thomson does in the 1849 Paper : We have a complete 

 theory of the heat engine, based on the Carnot cycle, 

 accounting for efliciencies which vary with temperature 

 differences, but requiring no definite mechanical equivalent 

 of heat ; nay, antagonistic to the existence of such an 

 equivalent. The caloric theory, as has been pointed out, 

 is quite consistent with the theoretical possibility of obtain- 

 ing an indefinitely great amount of mechanical energy from 

 any given quantity of heat, provided the letting dou'ii or 

 fall of level be indefinitely great. 



At the time we are discussing — 1S50 — the bare concep- 

 tion of the idea of an absolute zero of temperature is 

 one which is startling in its boldness ; and it must have 

 been difficult indeed then to imagine any definite line of 

 proof which could be follow'ed to establish the real exist- j 

 ence of such a physical limit. We are so familiar with 

 the existence of very high temperatures, vastlv transcend- 

 ing the temperatures in which we personally exist, that 

 we can hardly conceive a temperature limit on the ascend- 

 ing side ; that is, we can hardly think of any given high 

 temperature which could not in quite conceivable circum- 

 stances be exceeded. We know, for example, that any 

 metal — say platinum — may be melted if its temperature 

 be sufficiently increased ; that a further sulificient increase 

 will convert the liquid metal to the gaseous state, and 

 that the gaseous metal may be heated indefinitely while 

 in that state. We know the behaviour and properties of 

 many substances at high temperatures, and are aware 

 of the strong tendency of all chemical compounds, when 

 highly heated, to split up into the elementary bodies com- 

 posing them. All this we appreciate, but we find it 

 difficult to see how a point of temperature could be reached 

 when it could be said : This is a physical limiting point 

 on the ascending scale ; we may heat a substance up to 

 this temperature, but it is impossible to conceive of any 

 higher temperature. It is necessary here to distinguish 

 between a conceivable limit to an ascending temperature 

 and a practical limit under existing conditions. We may 

 thus place limits, say, to the temperature of coal-gas and 

 air explosions, or the temperatures possible from the electric 

 arc ; the limit with coal gas and air depending on one 

 set of conditions, and the electric arc upon another set, 

 such as the vapourising point of carbon, and so on. In 

 the same way, at the middle of last century it would have 

 been considered quite reasonable to suppose that human 

 existence was carried on at an intermediate plane of 

 temperature, and that temperatures might exist as low, 

 relatively to our mean temperature, as our known furnace 

 and combustion temperatures are high. At this time, no 

 doubt, such an idea was quite a reasonable one. 



No such limit could be proved, even by the aid of the 

 Carnot cycle, reasoning on the material theory of heat. 

 If we assume that heat is material, and that in some way 

 temperature fall doing work resembles, as Carnot sup- 

 posed, the fall of water doing work in passing from a 

 higher to a lower level, then no absolute zero is possible, 

 because the same quantity of heat is supposed to exist 

 at the low as at the high temperature. On this theory 

 nothing in the idea of temperature suggests a possible 

 physical limit. On the material theory, the notion of 

 temperature is one to which it is exceedingly difficult to 

 attach a precise meaning. 



Thomson's promises of further investigation were 

 fulfilled in 1S50, in which year he definitelv accepted the 

 dynamical theory of heat and finally abandoned the 

 material. His conclusions are given in a Memoir of the 

 first importance which was read before the Royal Society 

 of Edinburgh in 1.851. It was entitled " On the Dynamical 

 Theory of Heat." Before dealing with it, however, it is 

 desirable to consider the work of Joule and others on 

 another side of thermodynamics. 



Long before T850 the equivalence of mechanical work 

 and heat quantity had been accepted bv many scientific 

 men, and Rumford had, indeed, made measurements of a 



NO. 2030, VOL. 78] 



