HEAT FOR ADVANCED STUDENTS し CHAPTER I TEMPERATURE AND THERMOMETRY Temperature. -Our unaided senses suffice to distinguish between those conditions of a body which are designated by the terms hot and cold. It may, however, be remarked that the information thus acquired is of a somewhat relative character, A well-known experiment will illustrate this. EXPT. 1. Arrange three bowls, A, B, C, so that A contains cold water, B contains water that is somewhat warmer than that in A, whilst the water in C is warmer than that in either A or B. Immerse the right hand for some time in C, the left hand at the same time being immersed in A. Now plunge both hands into B; the water in this bowl will be felt to be cold by the right, and warm by the left hand. On a frosty day, a piece of metal which has been exposed to the air, will seem colder, when touched, than a piece of wood which has been similarly treated, although both the wood and metal are really equally cold. Nevertheless, our ideas of hotness and coldness are fundamentally derived from our sensations; and though we may have cause to distrust, in particular instances such as those given above, the information which we obtain through these channels, it does not follow that we should, even were it possible, discard entirely these foundations for our knowledge of the phenomena connected with the science of Heat. E B We may understand the expression "the temperature of a body" to mean its hotness (in the first place as determined by our sensations), compared with some standard temperature to be selected. A satisfactory definition of the meaning of the term "temperature" is at present impossible; the definitions which have been proposed either contain vague terms which render their use exceedingly unscientific, or else they define a state, of which we have some positive knowledge, in terms of certain assumptions which are really forced upon us by phenomena connected with the very condition which we seek to define. The latter form of arguing in a circle is to be particularly discountenanced. Unsatisfactory Definitions of Temperature.-A few remarks upon some definitions 1 of temperature, frequently met with, but fundamentally unsatisfactory, will illustrate the difficulty of expressing the meaning of temperature in exact words. 1. "The temperature of a body is the energy with which the heat in a body acts in the way of transferring or communicating a portion of itself to other bodies." This is a mere string of words, which may be learnt by heart, and may so give a feeling of knowledge gained. But upon analysis it is found to explain temperature-of which, as before remarked, we have some definite knowledge-in terms of "heat " (a hypothetical substance) and "energy" (a term which has a definite mechanical signification, but is here loosely applied, or misapplied). 2. "The temperature of a body is its power to communicate heat to other bodies." This definition, though simple, and therefore to be preferred to the above, is essentially bad: (a) in the use of the term power" in an unscientific sense, and (b) in introducing the assumption of something new and unknown, termed "heat," to explain temperature. "sensible Other similar definitions comprise such terms as heat." These are either unmeaning or erroneous. We are not directly sensible of heat, but of the temperature to which our skin is raised. At this stage, therefore, we may much more usefully under 1 For a further discussion of this point see Preston's Heat, p. 31. Macmillan and Co. stand by temperature merely the hotness of a body measured in a manner to be subsequently agreed upon. A great part of the science of Heat is occupied with settling the most trustworthy methods of measuring temperature. Some of these methods will be described in the present chapter. Thermoscopes.-A thermoscope may be defined as an arrangement which indicates the attainment of a certain arbitrary temperature. EXPT. 2.—Support a piece of thick sheet copper in a horizontal position on a tripod stand, and sprinkle small quantities of sulphur, lead, washing soda, paraffin wax, sealing wax, &c., &c., on it; then, if the copper be gradually heated by means of a Bunsen burner, it will be found that first one and then another of these substances will be melted. We might, therefore, when the paraffin wax melts in the foregoing experiment, say that the copper is at the temperature of melting paraffin wax, and so on. This method is actually used in certain scientific experiments. EXPT. 3.-It is well known that the dimensions of almost all bodies are altered by a change of temperature. A well-known experiment illustrates this. A brass ball which, when cold, will pass freely through a certain circular aperture, is found to be too large to do so when its temperature has been considerably raised. On cooling, however, it falls through the aperture when a certain temperature is reached, and we might use this fact to define a certain arbitrary temperature of the ball. A Thermometer is an Instrument designed to measure Temperature.--As a matter of fact, thermometers primarily indicate the temperature of the substances employed in their construction; but it is generally arranged that these substances should acquire the temperature of the medium in which they are placed. Any property of matter which varies continuously with the temperature might be used to measure temperature. Thus, linear expansion, cubical expansion or dilatation, the electrical resistance of a conductor, the thermo-electric force at the junction of the two unlike metals-all these and many other properties are used to measure temperature. In every case, |