| Table IV.—Glass (coloured). Common thickness 1mm·85. | ||
| Deviations of the galvanometer, |
Rays transmitted. | |
| ° | ||
| Deep violet | 18·62 | 53 |
| Yellowish red (flaked) | 18·58 | 53 |
| Purple red (flaked) | 18·10 | 51 |
| Vivid red | 16·54 | 47 |
| Pale violet | 16·08 | 45 |
| Orange red | 15·49 | 44 |
| Clear blue | 15·00 | 42 |
| Deep yellow | 14·12 | 40 |
| Bright yellow | 12·08 | 34 |
| Golden yellow | 11·75 | 33 |
| Deep blue | 11·60 | 33 |
| Apple green | 9·15 | 26 |
| Mineral green | 8·20 | 23 |
| Very deep blue | 6·88 | 19 |
It is sufficient to cast the eye rapidly over the second and third tables to be fully sensible of the truth of the proposition, that "the capacity which bodies possess of transmitting radiant heat is totally independent of their degree of transparency."
In fact, the liquid chloride of sulphur of a tolerably deep red brown transmits a considerably greater number of caloric rays than the fat oils of nut, the olive, and colza having a clearer tint; while these oils, although of a very decided yellow colour, are more permeable to radiant heat than several other liquids which are perfectly limpid, such as concentrated sulphuric and nitric acid, æther, alcohol, and water. The case is the same with solid bodies, among which we see sulphate of lime, citric acid, and other very diaphanous substances allow a much smaller quantity of heat to pass than some other bodies coloured or translucid, such as emerald, agate, tourmaline, borax, adularia, and sulphate of barytes.
But nothing is better calculated to demonstrate that transparency has little or no effect in the transmission of heat than a comparison of the effects obtained by the crystal of alum with those obtained by means of the smoky rock crystal. The table shows that, in respect to these substances as well as the others which we have just mentioned, the capacity to transmit radiant heat is inversely as the capacity of transmitting the rays of light. I was anxious to try how far this inverse ratio of the calorific to the luminous transmissions might extend, by varying the degrees of thickness so as to give to the light all the advantage and the whole of the loss to the caloric. We submitted to the test a plate of well-polished and perfectly transparent alum only one millimetre and a half in thickness, and a smoky rock crystal the thickness of which in the direction of its polished faces was 86 millimetres.
