60 CORRELATION OF PHYSICAL FORCES. 



uted to demonstrate this close analogy of heat and light, af- 

 ford a beautiful instance of the assistance which the progress 

 of one branch of physical science renders to that of another. 

 The discoveries of Oersted and Seebeck led to the construc- 

 tion of an instrument for measuring temperature, incompara- 

 bly more delicate than any previously known. To distin- 

 guish it from the ordinary thermometer, this instrument is 

 called the thermomultiplier. It consists of a series of small 

 bars of bismuth and antimony, forming one zigzag chain of 

 alternations arranged parallel to each other, in the shape of 

 a cylinder or prism ; so that the points of junction, which are 

 soldered, shall be all exposed at the bases of the cylinder : 

 the two extremities of this series ar,e united to a galvano- 

 meter that is, a flat coil of wire surrounding a freely-sus- 

 pended magnetic needle, the direction of which is parallel to 

 the convolutions of the wire. When radiant heat impinges 

 upon the soldered ends of the multiplier, a thermo-electric 

 current is induced in each pair ; and, as all these currents 

 tend to circulate in the same direction, the energy of the 

 whole is increased by the cooperating forces : this current, 

 traversing the helix of the galvanometer, deflects the needle 

 from parallelism by virtue of the electro-magnetic tangential 

 force, and the degree of this deflection serves as the index 

 of the temperature. 



Bodies examined by these means show a remarkable dif- 

 ference between their transcalescence, or power of transmit- 

 ting heat, and their transparency : thus, perfectly transparent 

 alum arrests more heat than quartz so dark coloured as to be 

 opaque ; and alum coupled with green glass Melloni found 

 was capable of transmitting a beam of brilliant light, while, 

 with the most delicate thermoscope, he could detect no indi- 

 cations of transmitted heat : on the other hand, rock-salt, the 

 most transcalescent body known, may be covered with soot 

 until perfectly opaque, and yet be found capable of transmit- 

 ting a considerable quantity of heat. Radiant heat, when 



