The Sun and Earth present an ongoing example of a<unk> -transfer process. Some of the Sun's thermal radiation strikes and<unk> s the Earth. Compared to the Sun, Earth has a much lower temperature and so sends far less thermal radiation back to the Sun. The<unk>  of this process can be quantified by the net amount, and direction (Sun to Earth), of energy transferred in a given period of time. In thermodynamics,<unk>  is energy in transfer to or from a thermodynamic system, by mechanisms other than thermodynamic work or transfer of matter. The various mechanisms of energy transfer that define<unk>  are stated in the next section of this article. Like thermodynamic work,<unk>  transfer is a process involving more than one system, not a property of any one system. In thermodynamics, energy transferred as<unk>  contributes to change in the system's cardinal energy variable of state, for example its internal energy, or for example its enthalpy. This is to be distinguished from the ordinary language conception of<unk>  as a property of an isolated system. The quantity of energy transferred as<unk>  in a process is the amount of transferred energy excluding any thermodynamic work that was done and any energy contained in matter transferred. For the precise definition of<unk> , it is necessary that it occur by a path that does not include transfer of matter. Though not immediately by the definition, but in special kinds of process, quantity of energy transferred as<unk>  can be measured by its effect on the states of interacting bodies. For example, respectively in special circumstances,<unk>  transfer can be measured by the amount of ice melted, or by change in temperature of a body in the surroundings of the system. Such methods are called calorimetry. The conventional symbol used to represent the amount of<unk>  transferred in a thermodynamic process is or . As an amount of energy (being transferred), the SI unit of<unk>  is the joule (J).
Heat
Cold
Chills
Inhalant
Toxicant
Allergen
Pollutant
Immunogen