NATURAL PHILOSOPHY. 175 



bodies in absorbing the rays of heat, and he had arrived at conclusions 

 which are quite new, and calculated to be oi' great importance in explaining 

 some of the great phenomena of nature. His experiments have been con- 

 ducted with the aid of a thermo-electric pile, which is far more sensitive to 

 the effects of heat than the most delicate thermometer, and the results he 

 had arrived at are, that the invisible rays of heat are absorbed in passing 

 th rough most transparent gaseous bodies, but that the luminous heat of the 

 sun is transmitted through them unimpeded. Mr. Tyndall repeated success- 

 fully some of these experiments, which require the most careful manipula- 

 tion. He first showed, by means of a galvanometer connected with the 

 thermo-electric pile, that rock-salt transmits the rays from a non-luminous 

 source of heat which are obstructed by glass ; and in the construction of his 

 apparatus he accordingly used the former substance. The apparatus con- 

 sisted of a tube four feet long and three inches diameter, closed at each end 

 with rock-salt, and so contrived that it might be exhausted of air, and other 

 gases substituted. Some fusible metal was kept heated at one end of the 

 tube, and at the other was placed a thermo-electric pile, which was connected 

 with a delicate galvanometer. Another sensitive pile was also heated by a 

 non-luminous body, and connected with the galvanometer, the indications 

 of which were, by an ingenious arrangement of the electric light, reflected 

 on a screen. The two sources of heat were so regulated as to neutralize each 

 other, and to bring the galvanometer needle to zero, when the tube contained 

 atmospheric air. When the tube was exhausted, and the rays of heat passed 

 through the partial vacuum, a decided deflection of the needle was observed. 

 The difference in the effect was, however, small when compared with the 

 brisk action of the needle when the tube was afterwards filled with coal-gas, 

 which absorbed the rays of heat much more than common air. Dr. Tyndall 

 having shown by these experiments which he said might be repeated with 

 similar effects with all other gases that the invisible rays of heat are vari- 

 ously absorbed by gaseous bodies, he next employed a source of heat com- 

 bined with light, resembling that of the sun. For this purpose he used the 

 oxyhydrogen light, the rays of which were passed through the tube when 

 filled with air; when exhausted, and when filled with coal-gas, in every 

 instance the effect was the same; for the luminous rays were not absorbed, 

 and the galvanometer needle, after having been brought to zero, remained 

 there. These experiments, Dr. Tyndall observed, have an important bear- 

 ing on the phenomena of nature; for they seem to explain how the planets 

 most distant from the sun may yet be sufficiently heated by its rays to 

 become habitable. Even the planet Neptune, though so remote from the 

 source of heat, may in the course of time have become heated by continually 

 receiving luminous rays, the heating portion of which, when not combined 

 with light, may be retained by the absorbing power of the atmosphere. In 

 short, the conclusions arrived at by Dr. Tyndall may be popularly stated as 

 follows: Obscure rays of heat that is, the rays of heat unaccompanied by 

 light are absorbed bypassing through the atmosphere, while those heat 

 rays which are luminous pass through it freely. Luminous solar heat, which 

 passes uninterruptedly through the atmosphere, on reaching the earth be- 

 comes changed into obscure heat, and is no longer capable of free atmos- 

 pheric transmission or radiation; therefore it is not sent back into space, but 

 remains in the atmosphere, serves to increase and preserve its temperature : 

 or, in other words, the atmosphere, acting in the same way as a ratchet- 

 wheel in mechanics, allowed the solar heat to come to the earth, and when 

 there, prevented it from radiating back again into space. 



