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the magnitude is the sum of heat and work; we are compelled to say either the sum of the heat and the heat-equivalent of the work, or the sum of the work and the work-equivalent of the heat.

Rankine has avoided this inconvenient mode of expression in his memoirs by assuming as his unit of heat the quantity which is equivalent to a unit of work. Nevertheless, although perfectly appropriate on theoretic grounds, it must be admitted that great difficulties oppose themselves to the general introduction of this measure of heat. On the one hand it is always difficult to change a unit when once adopted, and on the other there is here the additional circumstance that the heat-unit hitherto used is a magnitude intimately connected with ordinary calorimetric methods, and the latter being mostly based on the heating of water, necessitate only slight reductions, and these founded on very trustworthy measurements; the heat-unit adopted by Rankine, however, besides requiring the same reductions, assumes the mechanical equivalent of heat to be known,—an assumption which is only approximately correct. Accordingly, since we cannot expect the mechanical measure for heat to be universally adopted, we must always, when quantities of heat enter into an equation, first state whether these quantities are measured in the ordinary manner or by the mechanical unit, and consequently the above-mentioned inconvenience would not be removed by Rankine's procedure.

But

For this purpose, therefore, I will venture another proposition. Let heat and work continue to be measured each according to its most convenient unit, that is to say, heat according to the thermal unit, and work according to the mechanical one. besides the work measured according to the mechanical unit, let another magnitude be introduced denoting the work measured according to the thermal unit, that is to say, the numerical value of the work when the unit of work is that which is equivalent to the thermal unit. For the work thus expressed a particular name is requisite. I propose to adopt for it the Greek word

(epyov) ergon*.

* The author has used the German word Werk, which is almost synonymous with Arbeit, but he proposes the term Ergon as more suitable for introduction into other languages. The Greek word epyov is so closely allied to the English word work, that both are quite well suited to designate two

The processes which are considered in the mechanical theory of heat may be very conveniently described by means of this new term. Heat and ergon are, in fact, two magnitudes which admit of mutual transformation and substitution, without any alteration in the numerical values of the respective quantities being thereby involved. Accordingly, heat and ergon may, without preparation, be added to, or subtracted from, one another. When we consider the work produced during any change in the condition of a body, we must call it the ergon produced, if it be measured by the unit of heat, and here again we distinguish interior ergon and exterior ergon. The latter, as already stated in the memoirs, is dependent upon the entire series of successive changes, whilst the former is completely determined when the initial and final conditions, solely, are known. Assuming the initial condition to be given, therefore, the interior ergon may be regarded as a magnitude which depends solely upon the condition of the body at the moment under consideration.

Analogous to the expression thermal content of the body, we may introduce the expression ergonal content of the body. With reference to the last conception, however, the same remark applies which was previously made with reference to energy. We may understand by ergonal content, either the increment of ergon reckoned from a given initial condition, or the total ergonal content. In the latter case we have merely to conceive an unknown constant, having reference to the initial state, added to the increment of ergon; this is so obvious, however, that in such cases we may usually assume tacitly that the constant has been included.

The same remark also applies to the thermal content of a body. By this term we may likewise understand either the increment of heat calculated from an arbitrarily assumed initial condition, or the total thermal content. In the latter case a constant associated with that initial condition is to be added to the heat-increment. The only difference is that in the case of the ergonal content, the added constant is quite unknown, whilst

magnitudes which are essentially the same, but measured according to different units.-T. A. H.

in the case of the thermal content, the constant may be approximately determined, seeing that the absolute zero of temperature is to a certain extent known.

Now the quantity U is the sum of the thermal content and ergonal content, so that in place of the word energy, we may use if we please the somewhat longer expression, thermal and ergonal content.

In connexion with these remarks on Terminology I will venture another suggestion. Hitherto the heat which disappears when a body is fused or evaporated has been termed latent heat. This name originated when it was thought that the heat which can no longer be detected by our senses, when a body fuses or evaporates, still exists in the body in a peculiar concealed condition. According to the mechanical theory of heat, this notion is no longer tenable. All heat actually present in a body is sensible heat; the heat which disappears during fusion or evaporation is converted into work, and consequently exists no longer as heat; I propose, therefore, in place of latent heat, to substitute the term ergonized heat.

In order to distinguish, in a similar manner, the two parts of the latent heat which I have stated to be expended, respectively, on interior and on exterior work, the expressions interior and exterior ergonized heat might be used.

It must further be observed that of the heat which must be imparted to a body in order to raise its temperature without changing its state of aggregation (all of which was formerly regarded as free), a great portion falls in the same category as that which has hitherto been called latent heat, and for which I now propose the term ergonized heat. For, in general, the heating of a body involves a change in the arrangement of its molecules. This change usually occasions a sensible alteration in volume, but it may occur even when the volume of the body remains the same. For every change in molecular arrangement, a certain amount of ergon is requisite, which may be partly interior and partly exterior, and in producing this ergon, heat is consumed. Only a part of the heat communicated to a body, therefore, serves to increase the heat actually present therein; the remaining part constitutes the ergonized heat.

In certain cases, such as those of evaporation and fusion,

where the proposed term ergonized heat frequently presents itself, a more abbreviated form of expression may, of course, be adopted, should it be found convenient to do so. For instance, instead of using the expressions ergonized heat of evaporation, and ergonized heat of fusion, we may simply say, as I have done in my memoirs, heat of evaporation and heat of fusion.

APPENDIX B. (Page 239.)

ON THE SPECIFIC HEAT OF GASES AT CONSTANT VOLUME.

In the foregoing memoir it was stated that, in order to obtain the true heat-capacity of a substance, it must be used as a strongly over-heated vapour, and in fact in such a condition of expansion, that the vapour without appreciable error may be regarded as a perfect gas, and then its specific heat at constant volume must be determined. Now in reality this is not, strictly speaking, quite practicable, since permanent gases themselves, which are furthest removed from their point of condensation, I do not exactly follow the laws of a perfect gas; and hence we must certainly assume that at the temperatures at which they can be observed, condensible gases, and still more substances, which at the atmospheric pressure and at ordinary temperatures are either liquid or solid, and only become gaseous at higher temperatures, deviate still more considerably from those laws. To this must be added the circumstance that, with chemically constituted substances, and particularly with those of a complicated and not very permanent constitution, partial chemical changes accompany the processes of heating and cooling; such changes, even if they took place to so small an extent as to be with difficulty detected, might cause the quantities of heat taken up or given off by the gas during its heating or cooling at constant volume to deviate considerably from the true heatcapacity. Notwithstanding these imperfections, which are more or less unavoidable, the specific heat at constant volume corresponding to the gaseous condition of a body is always, of all the several specific heats of the substance, that which is most suited to serve as the approximate measure of the true heat-capacity, and consequently it is, in a theoretical point of view, a magnitude of some interest.

Now Regnault having recently determined experimentally the specific heats at constant pressures of a considerable number of gases and vapours, it was easy to calculate from these numbers, according to the principles of the mechanical theory of heat, the specific heats at constant volumes. Accordingly, immediately after the first publication of Regnault's results, I made these calculations and registered the results in a table for my own use. With reference thereto, it must be again remarked that the method of calculation employed is only strictly correct for a perfect gas; nevertheless the tables give at least approximate results for other gases. It must also be observed that the observation of the specific heat of a gas is the more difficult, and consequently the corresponding observation-number the less trustworthy the less permanent the gas is, and consequently the more its deportment deviates from the laws of a perfect gas. Since, then, no greater exactitude can be demanded from the calculation than that which the observation-numbers themselves possess, the method of calculation employed may be regarded as perfectly suited to its object. In forming my Table, I have thought it advisable to introduce a small change in one of the two series of numbers which Regnault has given for the specific heats at constant pressures. I have referred the numbers there to a unit somewhat different from that employed by Regnault, and I have likewise chosen a corresponding different unit in one of the two series containing specific heats at constant volumes. Regnault, in fact, gives us the specific heats of gases in two different ways. He first compares the weights of the gases, and states the quantity of heat which a unit-weight of each gas requires in order to have its temperature raised one degree; this he expresses in ordinary heat-units, that is to say, in terms of the quantity of heat which a unit-weight of water absorbs on being heated from 0° to 1o. In the second place he compares the volumes of the gases, and here he again uses the ordinary unit of heat; the volume to which the numbers refer being that which a unit-weight of atmospheric air occupies when it is of the same temperature and under the same pressure as the gas itself under consideration.

By this choice of units the second series of numbers clearly possesses a rather complicated signification, and its application

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