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1. In a paper which I lately communicated to the Edin­burgh Philosophical Journal, I endeavoured to show, that caloric flies off from the surface of a heated body by the re­pulsive energy existing between its own molecules, and con­sequently, that their velocity increases with the temperature of the body. This conclusion I deduced from the fact, dis­covered by Delaroche, that invisible caloric freely permeates very thin plates of glass, in the same manner as light, but that it is completely intercepted by thicker plates. If the tem­perature of the body be raised, the atoms of caloric will be brought nearer each other, their repulsive energy augmented, their velocity increased, and consequently, they will now find their way through a plate of glass which formerly intercepted them. If the temperature of the body be raised still higher, the molecules of caloric will acquire a velocity sufficient to permeate the various humours of the eye, and produce an impression on the retina, or in other words, they will become light. From this view of the subject, I was naturally led to the invention of an instrument which would be affected by visible caloric or light, whilst it would not be sensibly acted upon by invisible coloric, or heat. In short, I was led to the invention of a photometer, which appears to be the most accurate and delicate which has yet been described. But though such were the theoretical views which led to the invention of the instrument, its perfection does not depend upon any peculiar theory of light and heat. It is founded on the axiom, that equal volumes of air are equally expanded by equal quantities of light, converted into heat by absorption by black surfaces: and also on the well established principle that the quantity of light diminishes as the square of the distance of the luminous source from the object on which it is received.

VI. On a new photometer, with its application to determine the relative intensities of artificial light, &c