The tendency of heat to cause separation is not confined to increasing the distance between molecules, but acts also, no doubt, in increasing the distance between parts of the same molecule: in fact, the energy of heat is spent in pulling the constituent atoms asunder against the force of chemical affinity, as well as in pulling the molecules asunder against the force of cohesion, so that, at a very high temperature, it is probable that most chemical compounds would be decomposed, and many are so, even at a very moderate heat. Thus the attraction between oxygen and silver is so slight that at a comparatively low temperature the oxide of silver is decomposed. In like manner, limestone, or carbonate of lime, is decomposed when subjected to the heat of a lime-kiln, carbonic acid being given off, while quick-lime remains behind. Now, in separating heterogeneous atoms against the powerful force of chemical affinity, work is done as truly as it is in separating molecules from one another against the force of cohesion, or in separating a stone from the earth against the force of gravity. 77. Heat, as we have seen, is very frequently influential in performing this separation, and its energy is spent in so doing; but other energetic agents produce chemical decomposition as well as heat. For instance, certain rays of the sun decompose carbonic acid into carbon and oxygen in the leaves of plants, and their energy is spent in the process; that is to say, it is spent in pulling asunder two such powerfully attracting substances against the affinity they have for one another. And, again, the electric current is able to decompose certain substances, and of course its energy is spent in the process. Therefore, whenever two powerfully attracting atoms are separated, energy is spent in causing this separation as truly as in separating a stone from the earth, and when once the separation has been accomplished we have a species of energy of position just as truly as we have in a head of water, or in a stone at the top of a house.