784 First Linear Spectra of Emission of Mercury. 



It is improbable that the sodium plays more than an 

 indirect part in the generation of the gas. The following 

 suggestion seems at least probable. The presence of a trace 

 of sodium in aluminium renders it, as is well known, capable 

 of decomposing water, so that an electrode of such metal 

 becomes charged with hydrogen, probably chemically com- 

 bined as hydride, by mere exposure to air. The unknown 

 gas may be a modification of hydrogen like ozone is of 

 oxygen, capable of withstanding the discharge of an influence 

 machine, but decomposed by the discharge of a coil. This 

 theory is being investigated. 



The question of the quality of aluminium used for the 

 purposes of electrodes is of great practical importance. Of 

 old, aluminium was manufactured by a process which rendered 

 the presence of sodium unavoidable. A trace of sodium is, 

 however, a very undesirable impurity technically on account of 

 the metal being attacked by moisture, and the most strenuous 

 efforts of recent years have been directed to increasing the 

 purity of the metal for technical purposes, with the result 

 that the aluminium to-day differs entirely from that first 

 made. It is a very unsuitable material for the construction 

 of electrodes, the discharge passing with difficulty and irregu- 

 larity. Too great purity of the British aluminium may serve 

 to explain some of the difficulties besetting the X-ray bulb 

 manufacturers in this country. 



We are indebted to the Carnegie Trust for the provision of 

 the instruments used in this investigation. 



Physical Chemistry Laboratory, 

 The University, Glasgow. 



LXXVII. Gradual Modification of the First Linear Spectra of 

 Emission of Mercury. Preliminary Note by Prof. Enrico 

 Castelli *. 



[Plate XVII.] 



THE recent discoveries made by Sir W. Ramsay on the 

 transformation of several elementary substances, which 

 were lately mentioned at the Congress at Parma of the 

 Italian Association for the Progress of Science, have recalled 

 in my mind a fact I observed about a year ago, in studying 

 the luminous spectra of mercury. 



I had found that in taking (with a Steinheil spectrograph, 

 fitted with three quartz prisms) some spectrophotographs of 

 the electric arc in the vapour of mercury, contained in a 

 Uviol lamp, the spectrum lines (amounting only to 16, on 

 account of the limited dispersion of the apparatus), while 

 remaining constant in position, showed in time a gradual 

 variation of their photochemical action : the lines corre- 

 * Communicated by Sir Wm. Ramsay, F.E.S. 



