fact, everything leads us to believe that the equal layers which succeed one another in a diaphanous medium, act in the same manner on the rays of light which come in succession to pass through them, and that they consequently absorb or reflect a quantity of light proportional to the intensity of the incident rays; that is to say, that the loss sustained by the luminous radiation at every layer of equal thickness is constant. In the case under consideration, the invariable decrement of the light at each of the layers into which we suppose the screen divided is found to be none at all, or extremely feeble, because of the perfect transparency of the glass; and yet the caloric rays undergo in their successive passages an absorption, the sum of which is equal to about the half of their whole value; and the losses at each layer, instead of being constant, as happens to those sustained by the luminous rays, are found to differ enormously from one another, being in the proportion of the numbers 381, 71, 31 and 16.
The resistance of diaphanous media to the immediate transmission of the rays of heat is therefore of a nature altogether different from that which is presented by the same media to the propagation of light.
Whatever be the cause of this singular difference, it is highly important to determine with certainty whether it takes place at great distances from the surface at which the rays enter; and this may be done by repeating the experiments on layers of glass much thicker than those which we have been using.
With this view I took several pieces of the glass of Saint-Gobain, and caused them to be recast. This operation was not completely successful. The matter either formed itself into layers that were too thin, or was slightly striated. From among the thick pieces I selected that which was the purest. It was six inches in length. I divided it into three parts, of one, two, and three inches in thickness. The defects being uniformly distributed over all the points of the mass might probably enough alter the quantity of the caloric rays that would have passed through a perfectly pure mass of the same matter and thickness; but it is clear that they could have no influence on the nature of the progression of the losses which these rays might undergo in passing from one layer to another.
The following are the results obtained by exposing these screens to the ordinary radiation of 30°:
| Thickness of the screens in millimetres. |
Galvanometric deviations. |
| 27 | 17°·105 |
| 54 | 13·458 |
| 81 | 10·702 |
By a calculation exactly similar to that already made we find that,
