E. Linacre and B. Geerts
In earlier times the nature of heat was misunderstood as some type of substance with relatively large amounts of it in hot objects. By the same token, flammable objects were believed to contain a certain amount of matter, called phlogiston, which upon combustion vanished in the flames. These ideas were propounded by Stahl in 1703, and they flourished during the eighteenth century (1, p.109). The nature of temperature as heat intensity was first realised by Joseph Black in 1764 (2).
A problem that physicists and chemists ran into in the middle of the 19th century was that some chemical reactions implied that this substance had to have a negative weight. It was intuitive that matter must have weight, which may be negligible, but it cannot be negative. For instance, when phosphorus is burned (and thereby loses heat or phlogiston) a heavier oxide results.
Subsequently, the notion that heat equals matter was replaced by that of ‘caloric’, a weightless fluid which flows from hot to cold objects. Then experiments showed the connection between mechanical work and heat, and hence there arose the current theory of heat as the energy of vibrating molecules.
Temperature is a basic physical property, as are length and mass. However, it is ‘intensive’ whereas length and mass are ‘extensive’, i.e. adding two 10 cm lengths gives a length of 20 cm, but adding together two volumes of water at 20° C does not produce water at 40° C.