172 



HYDRATES IN AQUEOUS SOLUTION. 



at right angles to their respective axes of revolution. A plane parallel 

 plate of quartz M, was cemented in the lower end of tube D, and a similar 

 quartz plate was cemented to the inside of tube F, near the lower end 

 of this tube. The position of the latter quartz plate relative to the end of 

 the larger glass tube made it possible to slide the cell around on its lower 

 end as much as desired, without scratching the under polished surface of 

 the quartz, and without causing dust particles to adhere to the same. It 

 was necessary, of course, to use quartz in order to transmit the shorter, ultra- 

 violet waves. V shows a hard-rubber cylinder, carefully turned in a lathe 

 to fit neatly the outside of the tube D. The collar or flange of this cylinder, 

 when the cell was in adjustment, rested on the top of a larger glass tube in 

 such a position as to establish parallelism between the quartz plates M and 

 0. The smaller glass tube and the vulcanized ring were prevented from 

 ^ sliding along each other by a small amount of cement. It 



was very easy to scrape off this cement and to replace it 

 subsequently, whenever it became necessary or desirable 

 to alter the depth of the column of liquid between the 

 lower quartz plates. The quartz plate A was an essential 

 part of the cell. When it was not present, the passing of 

 the spark between the electrodes above the cell produced 

 a violent disturbance in the column of air in the tube D. 

 which resulted in the deposition on the quartz plate M of 

 the decomposition deposits of the electrode, i. e., the oxides 

 of cadmium, zinc, etc. Such a coating of foreign matter 

 on the quartz of course could not be permitted. Care 

 had to be taken to imprison dry air in the tube D, when 

 the plate A was cemented to the glass, because when this precaution was 

 omitted a thin layer of moisture was deposited on the inner or opposing 

 surfaces of the two quartz plates A and M, and produced spurious absorption 

 effects on the spectrogram. After three or four photographic exposures 

 with the spark had been made, the plate A invariably showed a thin whitish 

 coating of the material, having boundaries which suggested a projection of 

 the electrode on the quartz surface. This film was insoluble in water, and 

 it adhered very tenaciously to the polished quartz. It was easily removed, 

 however, by rubbing with a piece of soft muslin saturated with dilute hydro- 

 chloric acid. The coating was probably a mixture of the oxides of cad- 

 mium and zinc, driven hard against the quartz by the explosive violence 

 of the spark. N shows the absorbing liquid in the cell. The light passed 

 first through the quartz plate A, then through the dry air between A and 

 M, then through the quartz plate M, next through the solution N, and 

 finally through the quartz plate 0. When the cell was filled with dis- 



