REFRACTION AXD POLARISATION.] UNDULATORY FORCES. HE AT. 



29 



will pass through a plate of glass ; whilst that emanating 

 from a metal heated below redness, is entirely stopped in 

 its passage. Indeed, it would seem that heat passes 

 through solid transparent bodies just in proportion as it 

 is accompanied by light; for the heat of red-hot coke 

 will scarcely pass through thick glass ; whilst that pro- 

 duced by the electric light is transmitted to a large 

 extent. A plate of alum entirely prevents the trans- 

 mission of non-luminous heat. It has been found, by 

 the researches of Melloni and other philosophers, that 

 rock-salt is the most permeable to rays of heat, 

 allowing more than twice as many to pass through as 

 plate-glass. This substance is therefore chiefly employed 

 in experiments in connection with the refraction of heat. 

 An instrument of a very delicate kind is employed in 

 these experiments ; which we shall but briefly describe, 

 because the philosophy of its action will be fully investi- 

 gated under the head of Electro-Magnetism. 



If two metals, such as antimony and bismuth, are 

 arranged together in bars (one of each metal being 

 soldered to the other), and if wires are brought from the 

 last bar of each, so as to traverse over a magnetic needle, 

 a current of electricity will pass if either end of the 

 bundle of bars is heated : and a current thus gen- 

 erated by the force of heat, will, if present to but a 

 slight extent, affect the needle of a galvanometer, which 

 will be instantly diverted from its usual position. 

 This arrangement is much more sensitive of thermal 

 changes than either the mercurial or air thermometer, 

 and thus is invaluable in researches where small distur- 

 bance, of temperature have to be measured. It is called 

 the thermo-multiplier, and its arrangement is repre- 

 sented in Figs. 11 and 12. 



To illustrate the construction and use of the instru- 

 Plf. ll. mcnt, we give three en- 



gravings. In Fig. 11, we 

 observe the thermo-elec- 

 tric battery a consist- 

 ing of bars of antimony 

 and bismuth, from which 

 proceed two wires, n, p. 

 These are connected with 

 binding screws attached 

 to the galvanometer, re- 

 presented in oar next 

 illustration. 



The rays of heat being 

 . received at the end of the 

 arrangement of antimony 

 and bismuth bars, a cur- 

 rent of electricity is there- 

 by produced ; and if this is allowed to pass through a 

 wire coil, alternately wound over and under an ordinary 



Fif . IS. 



compass-needle, and yet not touching it, the electricity 

 will diverge the magnetic needle just in proportion to 



the strength of the current : and hence the amount of 

 radiant heat may be measured by the deflections of a 

 galvanometer. One of these is illustrated in Fig. 12, in 

 which we observe a magnetic needle suspended over a 

 coil placed just beneath it. The wires proceeding from 

 the thermo-electric battery, are observed on the left 

 hand of the engraving. 



When this instrument is employed for the purpose of 

 ascertaining the amount of heat radiating from a body, 

 or in researches on the laws of the refraction and polari- 

 sation of heat, the arrangement illustrated in Fig. 13, is 

 adopted ; wherein we observe, on the left hand, the source 

 of heat ; in the centre, are diaphragms, through which 

 the rays of heat may be passing ; and on the right hand 

 side are placed the thermo-electric battery, and the 

 galvanometer, by means of which the intensity of the 

 calorific rays is measured. 



As we have already remarked, this instrument is the 

 most sensitive 

 which we can em- 

 ploy for the pur- 

 poses we have 

 named; and it has 

 been of essential 

 service in all re- 

 searches having 

 for their object 

 the detection of 

 minute changes 

 of temperature. 



The mode usually adopted of decomposing light that 

 of the prism is well known ; and, by similar means, heat 

 may be divided into rays forming a spectrum, which is 

 highly analogous to one of light. A prism of rock- 

 crystal is used for this purpose, instead of one of glass. 



A vessel filled with boiling water is the source of he.tt 

 usually employed. The rays of heat radiated from it 

 are allowed to pass through a small hole in a screen, and, 

 impinging on a prism of rock-salt, they at once undergo 

 refraction, just as light would do by means of the ordi- 

 nary prism. 



The beautiful effects produced by the decomposition of 

 light, are of course visible to the eye ; whilst those of 

 heat are not only invisible, but are not perceived by the 

 senses at all. The thurino-multiplier, on being placed in 

 the heat-spectrum, at once evidences the refractive power 

 of the rock-crystal prism, and shows that the heat- 

 giving rays have an unequal refrangibility, just similar to 

 that of the rays of light. 



The discovery of this great similarity between heat 

 and light, is of the deepest importance in physical 

 science ; and, as we have already stated, it goes to prove 

 the truth of the undulatory theory of those forces. As 

 it completes their analogy in so many instances, the 

 mind is naturally led to infer that heat and light are 

 merely modifications of the same cause. 



The polarisation of heat may be effected in a very 

 similar manner to that of light ; and the effect takes place 

 when a calorific ray is allowed to pass through a plate of 

 tourmaline or mica. In certain positions of the polariser, 

 heat will pass through, whilst in others the heat is 

 entirely stopped. Wo sliall reserve a fuller description 

 of the result for the section on Polarised Light, because, 

 as that subject lias been so widely extended by the 

 researches of modem philosophers, we shall be better 

 able to trace both the facts and analogies of the same 

 properties in caloric. 



Heat can not only bo refracted by a prism of rock-salt; 

 but if a lens is made of that material, tho rays of heat may 

 be collected and focalised, and their effect made evident 

 by receiving them on the thermo-multiplier, although 

 they may be unaccompanied by light. Wu have here 

 another analogy with light in this singular property. The 

 simplest mode of trying this, is that of placing a lens of 

 rock-salt in front of a vessel containing boiling water, 

 having a screen before it. The end of the battery of the 

 multiplier must then be adjusted to the focal point of 

 the lens. The screen intercepting the rays of heat being 

 removed, the needle of the multiplier will immediately 



