XXII 



soon after diverted from conduction to radiation ; and his investiga- 

 tions on the action of gases and vapours upon radiant heat, continued 

 for twelve years, constitute, by reason both of the experimental skill 

 exhibited in their prosecution and the importance of the results 

 obtained, the crowning achievement of his life. The first indication 

 of the commencement of this work was given in a Friday evening 

 lecture, delivered in the Royal Institution on the 10th June, 1859, 

 " On the Transmission of Heat of different Qualities through Gases 

 of different Kinds." At this lecture the apparatus used throughout 

 his remaining investigations, with some modifications, was introduced 

 to his audience. It consisted of a tube having its ends stopped air- 

 tight by polished plates of rock-salt. This tube could be attached to 

 an air-pump and exhausted, so that any required gas or vapour could 

 be admitted into it. A thermo-electric pile being placed at one end 

 of the tube and a source of heat at the other, the needle of an ex- 

 tremely sensitive galvanometer connected with the pile was deflected. 

 After it had come to rest, the air WHS pumped from the tube and the 

 needle was carefully observed, to see whether the removal of the air 

 had any influence on the transmission of the heat. No such influence 

 showed itself, the needle remaining perfectly steady. A similar 

 result was obtained when hydrogen gas was used instead of air. 



It now occurred to the experimenter to increase the sensitiveness 

 of his apparatus by the use of a differential galvanometer. Under 

 the influence of two sources of heat, one of which was caused to pass 

 through the experimental tube, the astatic needle of the galvano- 

 meter was brought to zero by two powerful currents, which exactly 

 neutralised each other. A few strokes of the air-pump were now suffi- 

 cient to make the current from the thermo-pile at the end of the tube 

 to predominate over its antagonist by 40 or 50. On re-admitting the 

 air the needle again fell to zero, thus proving beyond doubt that the 

 air within the tube intercepted a portion of the radiant heat, the 

 source of heat being one at a temperature at about 300 C. Instead 

 of a differential galvanometer and two thermopiles, Tyndall after- 

 wards used only one pile with the two sources of heat operating upon 

 its opposite faces. The same method was applied to other gases, 

 with most remarkable results, gases being found to differ amongst 

 themselves with regard to their action on radiant heat as much as 

 liquids and solids do. Some gases he found bearing the same relation 

 to others that alum does to rock-salt. He found transparent and dry 

 coal-gas to be exceedingly powerful in cutting ofl the radiant heat 

 from a source at about 300 C., but when the lime light was placed at 

 one end of the tube and the rays concentrated by a convex lens were 

 sent through the tube, having previously been caused to pass through 

 a thin layer of water, the coal-gas had no po\ver to absorb the 

 luminous heat thus transmitted through it. He drew from these 



