230 Mr. J. B. Hannay's Examination of Vacua. 



for the propagation of radiation, and as the apparatus would 

 require to be heated to a high temperature, mercury as 

 an evacuating agent was abandoned, and fusible metal sub- 

 stituted. The " barometric height " of fusible metal is about 

 41 inches, so that all apparatus had to be constructed on 

 that scale. 



The fusible metal was composed of 50 per cent, bismuth and 

 25 per cent, tin and lead respectively; and its fusing-point was 

 94°. Increase of tin or lead raised the fusing-point; but a con- 

 siderable increase of bismuth (10 per cent.) only raised the 

 fusion-point a few degrees. A variation of 3 per cent, in any of 

 the constituents made little difference. About 30 lb. of the metal 

 was prepared. A portion was heated in a piece of combustion- 

 tubing over the blowpipe, but gave off no perceptible vapours. 

 Pure hydrogen was passed over 20 grms. of the metal in a 

 porcelain boat in a combustion-furnace at a red heat, but 

 after eight hours it had lost only 5 milligrammes, showing 

 that fusible metal is quite fixed, as the empty boat might 

 lose that weight Avhen heated so long. A portion was placed 

 in one end of a piece of combustion-tubing connected with a 

 Sprengel pump, the end containing the metal being slightly 

 elevated. The tube was then evacuated, and the portion away 

 from the metal heated until no more gas was given off, and 

 the MeLeod gauge showed an exhaustion of a millionth of an 

 atmosphere. The fusible metal was then gently fused and 

 allowed to run down to the end which had been heated. 

 The blowpipe was again applied and the metal strongly 

 heated; but it gave off no appreciable amount of gas. The 

 metal was therefore held to be free from dissolved gases: and 

 being found to be suitable for the work required of it, an 

 attempt was made to work a Sprengel pump with it. Although 

 its melting-point was 94°, it was not thoroughly liquid till 

 115° or thereby; so it was determined to work at a temperature 

 of from 120° to 150°, to ensure free flow of the metal. After 

 several experiments a bath was constructed as in fig. I. It 

 was made of strong sheet iron with double sides and three 

 sets of windows. Its dimensions are, length of bath 6 ft., length 

 of feet 10 in., inside space 10 in. by 8 in. It had a strong 

 iron tube with four arms running down one side, set in a step 

 at the bottom of the bath, and bolted to the top by a nut, so 

 that it could be removed and the glass apparatus attached for 

 experiment. It was found that, when fitted with a loosely 

 fitting lid, the heat from one of Fletcher's heaters was sufficient 

 when passing from below to maintain the interior at a tem- 

 perature of 300°. The windows were simply pieces of glass 

 made so as to be easily removed to gain access to the 

 interior. 



