Now one of the most interesting and important features of the process of energy transmission is that it usually provides the direct means for the identification of the acting energy. Energy, as it were, in movement, in the process of transmission, is capable of being detected in its different phases and recognised in each. The phenomena of transmission usually serve, either directly or indirectly, to portray the precise nature of the energy taking part in the operation. One of the most direct instances of this is provided by the transmission of heat energy. For illustrative purposes, let it be assumed that a body A, possessed of heat energy to an exceedingly high degree, is isolated within a spherical glass vessel CC, somewhat as already shown (Fig. 6). If it be assumed that the space within CC is a perfect vacuum, and that no material connection exists between the walls of the vessel and the body A, the latter is completely isolated, and no means whatever are available for the detection of its heat qualities (§ 30). It may seem that, if the temperature of the body A were sufficiently high, its energy state might be detected, and in a manner estimated, by its effect on the eye or by its luminous properties, but we take this opportunity of pointing out that luminosity is not invariably associated with high temperature. On the contrary, many bodies are found in Nature, both animate and inanimate, which are luminous and affect the eye at comparatively low temperatures. How then is the energy condition of the body to be definitely ascertained? The only means whereby it is possible to identify the energy of the body is by transmitting a portion of that energy to some other body and observing the resultant phenomena. Suppose, then, another body, such as B (Fig. 6), at a lower temperature than A, is brought into contact with A, so that a transmission of heat energy ensues between the two. The phenomena which would result in such circumstances will be exactly as already described in the case of the transmission of energy through a solid (§ 27). Amongst other manifestations it would be noticeable that the material of B was expanded against its inherent cohesive forces. Now if, instead of a spherical body such as B, a mercurial thermometer were utilised, the phenomena would be of precisely the same nature. A definite portion of the heat energy would be transmitted to the thermometer, and would produce expansion of the contained fluid. By the amount of this expansion it becomes possible to estimate the energy condition and properties of the body A, relative to its surroundings or to certain generally accepted standard conditions. Thermometric measurement is, in fact, merely the employment of a process of energy transmission for the purpose of identifying and estimating the heat-energy properties of material substances.