more diathermanous than the essence of turpentine; the same may be said of turpentine as compared with olive oil, and so on until we come to pure water; a liquid which, as it possesses the least power of refraction, possesses also the least power of transmitting heat. It is very true that, in the tables, glass appears almost as diathermanous as carburet of sulphur, although its refracting power is considerably less; but this equality is but in appearance; and to be convinced that it is so, we have only to recollect the manner in which the liquids have been subjected to the experiments. Before they can reach the liquid layer, the rays must have passed through the anterior face of the vessel containing it, and the glass gives but a transmission of from 21 to 22 for 35·3. Thus the radiation that will penetrate to the interior of the vessel will be of no greater force than this; so that even if the liquid transmitted all the rays that reached it, the quantity issuing from the recipient cannot exceed 22. This explanation is confirmed in a very striking manner by the transmissions of the chloride of sulphur and the protochloride of phosphorus. The indices of refraction of these two liquids, though not well known, are certainly higher than that of glass, and have different values; a fact from which it may be inferred with great probability that the quantities of transmitted heat are also different, though in the tables both these quantities appear equal to the transmission assigned to the carburet of sulphur.
There are, it is true, some real anomalies in the transmissions through balsam of copaiba and sulphuric æther. But the differences are very small, and may probably be referred to some slight error in the measure of the transmission or the refraction. The proportionality of these two elements is obvious, and so fully established in such a variety of cases, that it may hold as a general law for liquids, for the several kinds of glass, and probably for all those bodies which are without regular crystallization.
But this law totally fails with respect to crystallized bodies. We see, in fact, that carbonate of lead, a highly refractive and colourless substance, transmits less heat than Iceland spar and rock crystal, which are much inferior to it in refracting power; while rock salt, possessing the same transparency and the same index of refraction as citric acid and alum, gives six times their amount of calorific transmission.
The transparent and colourless bodies contained in the third table are nine in number, namely, rock salt, Iceland spar, rock crystal, topaz, carbonate of lead, sulphate of lime, citric acid, tartrate of potash and soda, and alum. These crystals transmit the following quantities of heat respectively:
92, 62, 54, 52, 20, 15, 12.[1]
- ↑ Such as have not a thermoscopic apparatus similar to that which we have employed may easily satisfy themselves that rock salt transmits almost all the
