60 CORRELATION OF PHYSICAL FORCES. 



uted to demonstrate this close analogy of heat and light, a 

 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 are 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 Mclloni 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 



