Heat, and the other elementary radiant principles, are often combined as the common cause of effects evident to our senses. The warmth of the solar rays, and their luminous influence, are not, however, commonly associated in the mind as the results of a single cause. It is only when we come to examine the physical phenomena connected with these radiations that we discover the complexity of the inquiry. Yet it is out of these very subtle researches that we draw the most refined truths. The high inferences to which the analysis of the subtile agencies of creation leads us, render science, pursued in the spirit of truth, a great system of religious instruction. Although we do not fear that heat and light can be confounded in the mind, so different are their phenomena,--we have heat rays, as from dark hot iron, which give no light, while in the full flood of the lunar rays the heat is scarcely appreciable by the most delicate instruments;--yet it is important to show how far these two principles have--been separated from each other. Transparent bodies have varied powers of calorific transparency, or transcalescence: some obstructing the heat radiated from bodies of the highest temperatures almost entirely even in the thinnest layers; whilst others will allow the warmth of the hand to pass through a thickness of several inches. Liquid chloride of sulphur, which is of a deep red colour, will allow 63 out of 100 rays of heat to pass, and a solution of carmine in ammonia, or glass stained with oxides of gold, or copper, rather a greater number; yet these transparent media obstruct a large quantity of light. Colourless media obstructing scarcely any light, will, on the contrary, prevent the passage of calorific rays. Out of every hundred rays, oil of turpentine will only transmit 31, sulphuric ether 21, sulphuric acid 17, and distilled water only 11. Pure flint glass, however, is permeated by 67 per cent. of the thermic rays, and crown glass by 49 per cent. The body possessing the most perfect transparency to the rays of heat is diaphanous salt-rock, which transmits 92, while alum, equally translucent, admits the passage of only 12 per cent.[43] Black mica, obsidian, and black glass, are nearly opaque to light, but they allow 90 per cent. of radiant heat to pass through them; whereas a pale green glass, coloured by oxide of copper,[44] covered with a layer of water, or a very thin plate of alum, will, although perfectly transparent to light, almost entirely obstruct the permeation of heat rays.