INTRODUCTORY. 9 



nals finally used were prepared by dipping twice in the molybdate solution 

 and then three times in the solution of uranium nitrate. 



It was also found that the terminals charged with various metals wore 

 away at quite different rates when the spark was passed. Those dipped 

 into a solution of a copper salt wore away at the rate of a millimeter or 

 more per minute, while those treated with molybdenum could be used for 

 hours without appreciable wear. The character of the spectrum given 

 by these molybdenum-uranium terminals may be seen from any of the 

 plates reproduced in the following chapters, which do not show complete 

 absorption in the ultra-violet. 



The coil used to produce the spark was a large Rontgen X-ray coil, 

 through the primary of which was passed an alternating current of from 

 5 to 8 amperes, 60 cycles; and across the secondary terminals was shunted 

 a capacity of about 0.011 microfarad. The spark used was about a centi- 

 meter in length, and was placed about 15 cm. above the slit, the direction 

 in which the spark passed being perpendicular to the length of the slit. 

 By this arrangement the grating received light from all parts of the spark 

 at the same time. In order to produce a uniform photographic strip of 

 the proper width, it was necessary to keep the spark terminals moving in 

 a direction parallel to the length of the slit, which was done by hand; a 

 suitable stand being used. Care was taken to move the spark-holder 

 always at the same rate, namely, a to-and-fro motion was executed in 

 about 4 seconds, which insured equality in times of exposure for the dif- 

 ferent strips of the spectrograms. The intensity of the spark undoubt- 

 edly varied somewhat, due to fluctuations of the voltage impressed upon 

 the primary terminals of the coil; but this was found to be so small that 

 no provision was made for regulating it. The Nernst filament is, however, 

 so sensitive to slight changes in voltage that a variable resistance was 

 placed in series with it; by regulating which the current was always kept 

 at 0.8 ampere during an exposure. 



MAKING A SPECTROGRAM. 



Ill making a spectrogram consisting of seven photographic strips with 

 a comparison spectrum, the following was the usual sequence of opera- 

 tions: Seven separate solutions were made up, the quantity of each being 

 usually 25 c.c. The cell to be used, having been cleaned and dried, was 

 filled to the required depth with the most concentrated solution of the 

 series, and the quartz plates limiting the depth of the solution adjusted to 

 parallelism. The exposure to the Nernst lamp was then made, the cur- 

 rent being kept at 0.8 ampere by hand regulation of the variable resist- 

 ance in series with it. The usual time of this exposure was 1 minute. An 

 opaque screen covering up the visible spectrum as far down as ^ 4000 was 

 then interposed betw r een the grating and the photographic film, and the 

 exposure to the light of the spark in the ultra-violet was made. The 

 duration of this exposure was usually 2 minutes. The photographic film 

 was then moved a distance of 6.5 mm. into the proper position for the 

 next exposure. The cell was emptied and rinsed out with a few drops of 

 the next solution, and the series of operations repeated for the second 



