8 ATLAS OF.IaBSORPTION SPECTRA. 



The current for the spark was obtained in the following manner: An 

 alternating e. m. f. of about io6 volts (133 cycles) was impressed on the 

 terminals of an induction coil of unknown ratio of turns. Eight or nine 

 amperes commonly flowed in the primary. The interrupter of the coil was 

 thrown out of circuit and the coil therefore performed the functions of a 

 transformer. In parallel with the secondary was placed a Le3fden jar about 

 18 inches high and of unmeasured capacity. No auxiliary spark was intro- 

 duced. The system could spark about 2.5 cm. in air between metallic 

 points. 



The great intensity of some of the lines characteristic of all the common 

 metals tried (Al, Cd, Cu, Fe, Pb, Zn, etc.) made these metals undesirable 

 for the present work. Cadmium and zinc were selected only because of the 

 strong continuous background to which they give rise. Uranium, its salts or 

 its earths were not used in this work because they are unmanageable. 

 Naturally the pure metal in air burns to oxide at once; pitchblende can not 

 be worked into a suitable shape (at least, for such specimens as we have 

 been able to obtain); and, pitchblende is so very heterogeneous that the 

 position of the spark can not be depended upon for an instant. To have 

 employed a neutral atmosphere in conjunction with a reciprocating mechanism 

 would have consumed, obviously, too much time and would have demanded 

 too complicated, cumbersome and inconvenient an assemblage of apparatus. 



THE CELL. 



In order to show the variations in the absorption spectrum of a given 

 substance when the thickness of the absorbing layer changed linearly, a 

 wedge-shaped cell was constructed. Vessels made on this principle have 

 been designed and used often before, notably by Angstrom, Gladstone, Govi, 

 Gibbs, Tumlirz, Hodgkinson, F. Melde, Hartley, and others.* Neverthe- 

 less, because the precise form of the cell is supposedly new and certainly 

 useful it may not be superfluous to enter into a detailed description of it 

 here. This little piece of apparatus was designed so that the relative posi- 

 tions of the quartz surfaces through which the light entered into, and emerged 

 from, the absorbing liquid could be varied at will, within certain limits. In 

 other words, matters were so arranged that the liquid could be in the form 

 either of a wedge, of variable angle, with zero thickness at the refracting 

 edge, or of a prism of variable angle and finite depth throughout, or of a 

 plane-parallel layer of changeable thickness. To satisfy these conditions it 

 was convenient to rely upon gravitation to preserve certain parts of the cell 

 in mutual contact. This in turn necessitated both the horizontal position of 



* See H. Kayser, " Handbuch der Spectroscopie, " v. in, pp. 58, 59. 



