The word “heat” is frequently present in the people life, for example, when we speak that the weather is hot: it is very hot today! However, few peoples understand the truly meaning of this word and how its concept was discussed along of the 18th century.
In the beginning of the 18th century, there were two hypotheses about the origin of the heat. The hypothesis more accepted regarded the heat like indestructible substance “occupying the pores” of the bodies and that it would flow from a body “hot” to another “cold”. The scientist Lavoisier called this substance of "caloric". This hypothesis meant that the heat could be transferred from a body to another one, but the total amount of heat was conserved.
The rival hypothesis, in which two authors was the philosophers Francis Bacon and Robert Hooke, explained the heat as being a minuscule motion of vibration of the particles of bodies. Both theories about the heat concept explained some everyday examples that there was in that century, like rub two kindling and feel both getting hot. Nevertheless, the caloric theory suffered a lot of difficulties in another examples, like we will mention below.
According to first theory, the caloric was a substance that would occupy the pores of the bodies and because that it would have some weight that could be measured, at least in principle. In the 18th century already had methods for such measurement to be realized, even if the weight were so small. The scientist adventurer Benjamin Thomson realized the following experiment: he weighted a large piece of bronze and after that he pierced the piece of bronze in order to generate very heat by friction. With that he would expect that after piercing, the piece of bronze would be lighter than before. However, it was not what happened and thus he came to believe that the caloric was something not constant but an unlimited substance within the material. Therefore the caloric theory was discarded, leading other scientist at that time to defend the theory of heat as being a vibratory motion of the particles of bodies.
In the 19th century, the scientist James Watt developed the steam machine, so showing that the heat could be converted to energy to put in moving from simple things to train. The steam machine definitely showed, together with some others experiments, that the heat is nothing more than a type of energy. Thus, if we use the concept of heat as being the vibratory motion of the particles of bodies we can say: the heat is, of course, a type of energy; however, it is an amount of energy that is being transferred from a body to another one. This transfer happens through of collisions of particles of a body with particles of another one. Therefore a hot body is a body with more vibratory motion than a cold body.
Although nowadays the word “heat” is present in any discussion, it generated a great scientific and philosophic controversy in the 18th and 19th century. The concept of heat is very well established now, being extremely used by physicists and engineers
