PllESIDENTIAL ADDRESS. 813 



' 8. The extremely important discoveries recently made by Mr. Joule, of Man- 

 chester, that heat is evolved in every part of a closed electric conductor moving 

 in the neighbourhood of a magnet, and that heat is generated by the friction o^f 

 fluids in motion, seem to overturn the opinion commonly held that heat cannot 

 be generated, but only produced from a source where it has previously existed 

 either in a sensible or in a latent condition. In the present state of science 

 however, no operation is known by which heat can be absorbed into a body with- 

 out either elevating its temperature or becoming latent, and producing some 

 alteration in its pliysical condition ; and the fundamental axiom adopted by 

 Carnot may 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 founda- 

 tion when our experimental data are more complete. On this understandino-, and 

 to avoid a repetition of doubts, I shall refer to Garnet'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 efficiencies which vary with temperature differences but 

 requiring no detiiiite 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 obtaining an indefinitely 

 great amount of mechanical energy from any given quantity of heat, provided the 

 letting doicn or fall of level be indefinitely great. 



At the time we are discussing— 18.50— the bare conception 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 followed to establish the real existence of' such a physical limit. We are so 

 familiar with the existence of very high temperatures, vastly transcending the 

 temperatures in which we personally exist, that we can hardly conceive a 

 temperature limit on the ascending side ; that is, we can hardly think of any 

 given high temperature which could not under quite conceivable circumstances 

 be exceeded. We know, for example, that any metal— say platinum— may be 

 melted if its temperature be sulficiently increased; that a further sufficient 

 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 composing 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. AVe may thus place 

 limits, say, to the temperature of coal-gas and air explosions, or the temperatures 

 possible frona 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 com- 

 bustion 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, reasonino- 

 on the materia] theory of heat. If we assume that heat is material, and that in 

 some way temperature fall doing work resembles, as Carnot supposed, the fall of 

 water doing work in passing from a higher to a lower level, then no absolute zero 



