Heat as a Quantity

Heat, therefore, may pass out of one body into another just as water may be poured from one vessel into another, and it may be retained in a body for any time, just as water may be kept in a vessel. We have, therefore, a right to speak of heat as a measurable quantity, and to treat it mathematically like other measurable quantities so long as it continues to exist as heat. We shall find, however, that we have no right to treat heat as a substance, for it may be transformed into something which is not heat, and is certainly not a substance at all—namely, mechanical work.

We must remember, therefore, that though we admit heat to the title of a measurable quantity, we must not give it rank as a substance. Instead, we must hold our minds in suspense until we have further evidence as to the nature of heat.

Such evidence is furnished by experiments on friction, in which mechanical work—instead of being transmitted from one part of a machine to another—is apparently lost. At the same time, and in the same place, heat is generated, with the amount of heat being in an exact proportion to the amount of work lost. We have, therefore, reason to believe that heat is of the same nature as mechanical work; that is, it is one of the forms of Energy.

In the eighteenth century, when many new facts were being discovered relating to the action of heat on bodies, and when at the same time great progress was being made in the knowledge of the chemical action of substances, the word Caloric original: "Caloric" — a historical scientific theory that heat was a self-repelling fluid. was introduced to signify heat as a measurable quantity. So long as the word denoted nothing more than this, it might be usefully employed. However, the form of the word accommodated itself to the tendency of the chemists of that time to seek for new "imponderable substances" substances that have no detectable weight, such as light or electricity were once thought to be.. Consequently, the word caloric came to connote1 not merely heat, but heat as an indestructible, imponderable fluid, insinuating itself into the pores of bodies, dilating and dissolving them, and—

1. "A connotative term is one which denotes a subject and implies an attribute." — Mill’s Logic, book i. chapter ii. section 5.