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Updated: May 8, 2025
It was simply to secure more complete mastery over the position of the focus, so as to cause it to fall accurately upon the diamond, that the mode of experiment here described was resorted to. Ultra-red Rays: Calorescence. In the path of the beam issuing from our lamp I now place a cell with glass sides containing a solution of alum. All the light of the beam passes through this solution.
The important part played by these ultra-red rays in Nature may be thus illustrated: I remove the iodine filter, and concentrate the total beam upon a test tube containing water. It immediately begins to splutter, and in a minute or two it boils. What boils it? Placing the alum solution in front of the lamp, the boiling instantly ceases.
We know now that the chief part of heat comes from the ultra-red rays that show no light; and the main part of the chemical changes in the vegetable world are the results of the ultra-violet rays at the other end of the spectrum, which are equally invisible to the eye, and are recognized only by their potent effects.
Determining in the first place the total emission from the electric lamp, and then, by means of the iodine filter, determining the ultra-red emission; the difference between both gives the luminous emission. In this way, it is found that the energy of the invisible emission is eight times that of the visible.
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