instance of this in the reactions which take place without the aid of external energy; and again, in those very numerous cases which, such as the combustion of hydrogen and carbon, or the decomposition of explosives, the reactions continue when once primed. I may make a further observation apropos of thermal and photic energy. These are not two really and essentially distinct forms, as was thought in the early days of physics. When we consider things objectively, there is absolutely no light without heat; light and heat are one and the same agent. According as it is at this or that degree of its scale of magnitude, it makes a stronger impression on the skin (sensation of heat) or on the retina (sensation of light) of man and animals. The difference may be put down to the diversity of the work and not to that of the agent. The kinetic theory shows us that the agent is qualitatively identical. The words heat and light only express the chance of the meeting of the radiant agent with a skin and a retina. At the lowest degree of activity this agent exerts no action on the terminations of the thermal cutaneous nerves, nor on the optic nerve-terminations. As this degree is raised the former of these nerves are affected (cold, heat) and are so to the exclusion of the nerves of vision. Then they are both affected (sensation of heat and light), and finally, beyond that, sight alone is affected. The transformation of one energy into the other is therefore here reduced to the possibility of increasing or decreasing the intensity of the action of this common agent in the exact proportions suitable for passing from one of the conditions to the other; and this is easy when it is a question of going up the scale in the case of light, and, on the contrary, it is not realizable
