382 THIRD RT-PORT— 1833. 



the ratio of transmission for the heat of a lamp to be propor- 

 tional to their refractive powers ; but in crystallized solids the 

 strangest anomalies appeared. The most remarkable of these 

 was, that the transmissive power of rock-salt for heat was six or 

 eight times greater than that of an equal thickness of alum, 

 which had almost the same transparence and refractive powers. 

 But the extraordinary property discovered by M. Melloni in 

 rock-salt is this, that it is equally transparent for heat of all 

 temperatures or of all degrees of refrangibility ; that the heat 

 of the hand loses as little in passing through a given thickness 

 of it, as the same quantity of solar or any other kind of heat. 

 This singular fact places rock-salt in a still more important posi- 

 tion in relation to heat than the discovery of Sir David Brewster 

 has already placed it in regard to light. 



A plate of rock-salt 7"™ ('28 in.) in thickness transmits equal 

 proportions of heat, radiating from red-hot iron, from the flame 

 of oil or alcohol, boiling water, and water even below 120°Fahr. ; 

 and in all these cases the transmission was 92 per 100 of the in- 

 cident rays. A piece about an inch in thickness was equally 

 constant in the ratio of transmission. 



To all who have been in the slightest degree occupied with 

 the study of radiant heat, the value of this property will at once 

 be apparent. It gives the means of making experiments 

 hitherto physically impossible. The more delicate of M. Mel- 

 loni's experiments are made with the thermo-multiplier ; but 

 many are capable of repetition by common instruments. The 

 different transmissibility of alum and salt, for example, may be 

 shown with the rudest apparatus. By means of a common three- 

 sided prism, formed of rock-salt, M. Melloni can refract a pen- 

 cil of heat derived from boiling water. 



The following is an abstract of the later discoveries of 

 M. Melloni, communicated by him to Professor Forbes, in 

 order to be laid before the Association. 



The colouring matter of glass has been proved to diminish its 

 power of transmitting heat. The question is. Does this ab- 

 sorptive force stop all the radiant heat, except that having a 

 definite degree of refrangibility, in the same way as it acts on 

 the rays of light ? 



The character of colour, which always accompanies and 

 indicates the refrangibility of light, is wanting in regard to 

 heat ; and the operation of measuring the intensity of an 

 emergent pencil of the latter would be, it is easy to see, 

 extremely diflicult. It has formerly been shown that the power 

 of heat to traverse diaphanous bodies diminishes with the 



