The engagements resulting from the establishment of the Pneumatic Institution, and from a course of experiments, to which I have been obliged to pay great attention, have prevented me from acknowledging to you my obligations for the very great pleasure I received from your last excellent letter.
In experiments on Light and Heat, we have to deal with agents whose changes we are unable directly to estimate. The most we can hope for is such an arrangement of facts as will account for most of the phenomena.
The supposition of active powers common to all matter, from the different modifications of which all the phenomena of its changes result, appears to me more reasonable than the assumption of certain imaginary fluids alone endowed with active powers, and bearing the same relation to common matter, as the vulgar philosophy supposes spirit to bear to matter.
That the particles of bodies must move, or separate from each other, when they become expanded, is certain. A repulsive motion of the particles is directly the cause of expansion; and when bodies are expanded by friction, under circumstances in which there could be no heat communicated by bodies in contact, no oxidation and no diminution of capacity, I see no difficulty in conceiving the repulsive motion generated by the mechanical motion.
Your excellent and truly philosophic observations will induce me to pay greater attention to all my positions. It is only by forming theories, and then comparing them with facts, that we can hope to discover the true system of nature. I will endeavour very soon to give an answer to the remaining part of your excellent letter.
I have now just room to give you an account of the experiments I have lately been engaged in, though they are not much connected with light and heat.
First.—One of Mr. William Coate's children accidentally discovered that two bonnet-canes rubbed together produced a faint light. The novelty of this phenomenon induced me to examine it, and I found that the canes on collision produced sparks of light, as brilliant as those from the flint and steel.
Secondly.—On examining the epidermis, I found, when it was taken off, that the canes no longer gave light on collision.
Thirdly.—The epidermis, subjected to chemical analysis, had all the properties of silex.
Fourthly.—The similar appearance of the epidermis of reeds, corn, and grasses, induced me to suppose that they likewise contained silex. By burning them carefully, and analysing their ashes, I found that they contained it in rather larger proportions than the canes.