682 PROCEEDINGS OF THE AMERICAN ACADEMY. 



range of this electrical stimulus, run through a gamut of dissocia- 

 tions and recombinations. One never deals with a strictly pure vapor 



or gas. 



The spectra of metals in atmospheric air are the visible evidence of 

 extremely complicated chemical reactions due to the reaction of the 

 metallic vapor and the gases of the atmosphere. The spectra of gases 

 also in narrow containing vessels of glass or of quartz, are modified by 

 the walls of these vessels when the temperature of the gases is very 

 high ; moreover, the ordinary method of obtaining photographic spectra 

 either of metals in air or rarefied gases by long continued discharges 

 produced by the Ruhmkorf coil of transformers masks certain funda- 

 mental reactions. 



It is therefore desirable to study the effect of known quantities of 

 energy successively applied to produce spectra either of metals or gases. 

 This can best be accomplished by charging a condenser to a known 

 amount by a known electromotive force, and by discharging the con- 

 denser between terminals of metals either in air or in gases. If the 

 spectra produced in this manner by discharges varying from one to any 

 desired number are photographed on the same plate and treated alike 

 in the same developer, the ground may be prepared for some generaliza- 

 tion of the extremely complicated reactions I have mentioned ; I believe 

 that this method is a fundamental one to use if order is to be brought 

 out of the chaos of spark spectra. 



I have applied this method in the following manner : A storage bat- 

 tery of from ten thousand to twenty thousand cells is employed to charge 

 a condenser — .1 to .3 microfarad. By a simple mechanical appliance 

 the condenser is detached from the poles of the battery, and is discharged 

 between suitable terminals. Although it is impossible to avoid a slight 

 spark at the moment of making contactrwith the receiving system, an 

 approximately equal quantity of electricity is communicated to this system 

 at each discharge. The method also permits of a definite control. 



In order to photograph on the same plate the spectra produced by 

 successive discharges the photographic plate must be slid vertically from 

 one position to another, at the focus of a Rowland grating. The most 

 convenient arrangement for the study of gaseous spectra is to employ a 

 grating of short focus, and to enclose it in a light, tight box. For com- 

 pactness, and to dispense with a dark room, I have employed the method 

 of mounting the grating in such a manner that the normal to the ruled 

 surface passes through the slit. The camera swings on the arc of a 

 circle described by an arm of half the radius of the grating. This arm 



