32 M. MELnONI ON THE FREE TRANSMISSION 



more diathormanous 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 refrac- 

 tion, 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 sub- 

 jected to the experiments. Before they can reach the liquid layer, the 

 rays must have passed through the anterior face of the vessel contain- 

 ing 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 can- 

 not 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 aether. But the differences are very 

 small, and may probably be referred to some slight eruor in the measure 

 of the transmission or the refraction. The proportionality of these two 

 elements is obvious, and so fully established in svich 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 crystal- 

 lization. 



But this law totally fails with respect to ciystallized bodies. We see, 

 in fact, that carbonate of lead, a highly refractive and colourless sub'* 

 Stance, 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 tlie 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.* 



* 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 



