"The production of heat alone is not sufficient to give birth to the impelling powerː it is necessary that there should also be cold; without it the heat would be useless. And in fact, if we should find about us only bodies as hot as our furnaces... What should we do with it if once produced? We should not presume that we might discharge it into the atmosphere... the atmosphere would not receive it. It does receive it under the actual condition of things only because.. it is at a lower temperature, otherwise it... would be already saturated."