Popular scientific lectures

at the second body is cooled. We may say that the first body is warmed at the expense of the second bod

pid equalisation of the temperatures, that the respective changes of temperatures u and u' are invers

m'u

the constancy of the sum of these products in simple processes of conduction. If a quantity of heat disappears at one point, an equally large quantity will make its appearance at some other point. The retention of this idea leads to the discovery of specific heat. Black, finally, percei

ception of heat is now as unessential as was formerly its conception as a substance. Both ideas were favored or impeded solely by accidental historical circumstances. It does not follow that heat is not a substance from the fact that a mechanical equivalent exists for quantity of heat. We will make this clear by the following question which bright students have sometimes put to me. Is there a

there is no mechanical equivalent of quantity of water, but there is a mec

ntity of electricity disappears as work is done, but we simply assume that the electricities come into

electricity? The reason is purely historical, wholly conventional, and, what is stil

, at a certain distance r of their centres, a certain repulsion p. We bring into contact with B now a ball C, suffer both to be equally electrified, and then measure the repulsion of B from A and of C from A at the same distance r. The sum of these repulsion

uantity and C be the capacity, is proportional to Q2/2C, or, more simply still, to the energy of the charged jar. If, now, we discharge the jar completely through the thermometer, we obtain a certain quantity of heat, W. But if we make the discharge through the thermometer into a second jar, we o

nged and its value by Riess's thermometer is decreased.

ity contained in a jar should be measured by the heat produced in the thermometer? But then, this so-called quantity of electricity would decrease on the production of heat or on the performance of work, whereas it now remains unchanged; in th

ater for work were called quantity, and had to be measured, therefore, by a mill instead of by scales; then this quantity also would disappear as it performed the work. It may, now, be easily conceived tha

its quantity disappears when work is done. But that heat is not a substance follows from this as little as does the opposite conclus

have to assume that that which we call quantity of heat was the energy of a substance whose quantity remained unaltered, but whose energy

fectly indifferent and possesses not the slightest scientific value, whether we think of heat as a substance or not. The fac