Radiation by Rarefied Gases, 503 



tube. The joints were made air-tight by means of sodium 

 silicate, which proved to be a highly useful cement, and did 

 not give rise to any impurities. Short lengths of tubing 

 containing mercury were placed round the platinum wires to 

 convey the current to the electrodes. 



The behaviour of four gases only was investigated, viz. 

 hydrogen, oxygen, nitrogen, and carbonic oxide. Hydrogen 

 and oxygen were prepared by electrolysis of pure newly- 

 distilled water acidulated with phosphoric acid. The nitrogen 

 was obtained by passing pure air over heated copper-turnings 

 reduced by hydrogen. Carbonic oxide was prepared by the 

 reaction of sulphuric and oxalic acids, and purified by passing 

 through caustic potash. In producing these gases all rubber 

 tubes w r ere dispensed with, and the different parts of the 

 generating apparatus were soldered together. 



The discharge-tabe was connected through a Kundt glass 

 spring and a set of cleaning-tubes to the tube used for intro- 

 ducing the gas, a mercury-valve, and the air-pump. The 

 mercury-valve consisted of a U-tube communicating at the 

 bottom with a long tube full of mercury. By varying the 

 level of the mercury by means of a reservoir the valve could 

 be opened and closed. The tube for introducing the gases 

 corresponded in the main to Cornu's arrangement. A vertical 

 glass tube is filled with mercury whose level can be varied by 

 means of a reservoir connected through a flexible tube, as in 

 the case of the mercury- valve. At a point some distance 

 from the bottom is attached a capillary U-tube, the end of 

 which, in the process of filling, is introduced into a small 

 reservoir containing the gas. Lowering the mercury esta- 

 blishes a connexion with the discharge-tube through the 

 drying-tubes, and on raising the level the gas is shut off 

 from the atmosphere. 



The current was furnished by a battery of 800 small Plante 

 accumulators, regulated by means of a liquid resistance con- 

 sisting of cadmium iodide dissolved in amyl alcohol, and 

 measured by a dead-beat reflecting-galvanometer. The fall 

 of potential in the discharge-tube was measured by a Mascart 

 quadrant-electrometer. 



The bolometer used for the experiments consisted of two 

 gratings cut out of tinfoil mounted in ebonite frames. These 

 frames were placed one behind the other in a tube with 

 double walls, the posterior one being protected from radiation 

 by a small double screen. Four diaphragms were mounted 

 in the tube in front of the gratings, to diminish air-currents. 

 The grating absorbing the radiation occupied a circular space 

 of 16 millim. diameter. It was blackened by precipitated 



