Ront-gen Radiation from Gases and Vapours. 665 



and after a balance has been obtained between the two gases, 

 one is placed over the upper window (d) of each of the gas- 

 chambers. If the radiation from both chambers is of the 

 same character, it will be equally absorbed, and hence the 

 balance will remain undisturbed. This method is independent 

 of the hardness of the bulb, since the absorption takes place 

 simultaneously for the radiation from both gases. 



Similarly by filling both gas-chambers with air and placing 

 the screens, the one over the upper window (d) of the one 

 chamber, and the other over the side window (c) of the second 

 chamber, it was possible to test whether the secondary 

 radiation from air was of the same character as the primary 

 producing it. This was found to be the case, whatever the 

 hardness of the primary rays. 



Materials, fyc. 



It may be desirable to add a few words on the methods of 

 preparation, and introduction of the different gases and 

 vapours employed. 



Hydrogen. — This was prepared from pure zinc (guaranteed 

 free from arsenic, antimony, &c.) and dilute sulphuric acid 

 also guaranteed pure. The zinc with a little distilled water, 

 which had 1 een previously boiled, was placed in a TToulfY's 

 flask fitted with a dropping funnel and exit tube, the latter 

 being connected through the drying-tubes and cotton-wool 

 plug to the gas-chamber. The whole was then made air- 

 tight, and pumped out with the water-pump to a low pressure. 

 A little of the dilute acid (also previously boiled for some time 

 to expel dissolved air) was then run in through the dropping 

 funnel, and enough hydrogen produced to bring the pressure 

 to atmospheric again. The apparatus was then again pumped 

 out and the process repeated four or five times. In this way 

 the apparatus was filled with very pure gas. A subsequent 

 analysis of the gas in the apparatus by the usual eudiometric 

 method failed to reveal any measurable amount of impurity. 



Helium. — This gas was obtained by heating very finely- 

 powdered thorianite in a porcelain tube, and passing the gas 

 evolved over solid caustic potash to absorb any carbon dioxide 

 present, and drying with phosphorus pentoxide in the usual 

 way. Mr. Burton, who has recently had occasion to prepare 

 large quantities of helium, informs me that the gas obtained 

 in this way is very pure. 



Ammonia was obtained from a cylinder of liquid ammonia. 

 The gas was subsequently analysed and found to be pure. 



Carbon dioxide was prepared from pure marble and pure 

 dilute hydrochloric acid in an ordinary kip. In every case 



