418 Prof. R. Bunsen's Spectral- Analytical Researches. 



the experiments upon which the measurements are based is 

 doubtful. On attempting to reduce to a common scale the 

 spectra obtained by different observers working with different 

 dispersive powers, different breadths of slit, now at high, now at 

 lower temperatures, Tables are obtained which are altogether 

 useless for laboratory purposes. In order to obtain a sure stand- 

 ing-ground, preliminary researches into the purity of the mate- 

 rial employed for fundamental observations are necessary, inas- 

 much as doubts about individual lines can only be set at rest 

 when it is known that the material employed is perfectly pure. 

 Graphic representations of spectra are valueless for practical work, 

 if every uncertainty with regard at least to the principal lines 

 is not removed. The methods employed in the following research 

 for the production of perfectly pure materials are, where it ap- 

 peared necessary, discussed in detail. 



I. Battery and Spark-apparatus. 



No one who is in the habit of using currents of great intensity 

 with temporary interruptions of days, weeks, or months, but must 

 have experienced much inconvenience from setting up, taking to 

 pieces, and cleaning the constant batteries which are ordinarily 

 employed. So long as the production of spark-spectra continues 

 to involve such tedious and troublesome operations, it is not to 

 be expected that these spectra will be generally used in chemical 

 laboratories ; the battery about to be described (which is without 

 clay cells) is designed to do away with these difficulties. 



I have already shown that a mixture of potassium dichromate 

 and sulphuric acid may advantageously replace the nitric acid of 

 the carbon-zinc battery without clay cells. More recently Leeson 

 and Warrington have proposed to use this mixture in a battery 

 with clay cells, in such proportions as shall ensure the production 

 of chrome alum (by the action of the sulphuric acid upon the 

 dichromate) and the solution of this salt in the water present. 

 Such a liquid is produced by mixing together — 



Potassium dichromate . . . . 1'33 part. 

 Concentrated sulphuric acid 1*0 „ 



Water 6*0 parts. 



This mixture, however, is not so advantageous in batteries 

 without clay cells. According as the green di-acid or the blue 

 tri-acid modification of chromium oxide is produced, the electro- 

 lytic decompositions which occur may be arranged in one or 

 other of the following schemes, in which the original constituents 

 are placed to the left, and the products of electrolytic decompo- 

 sition to the right : — 



