180 HYDRATES IN AQUEOUS SOLUTION. 



nating current necessitated curving the edges of the electrodes, so that 

 they were somewhat closer together at the middle than at the ends. Due 

 to the tearing away of the metal and to various other causes, the innumer- 

 able thread-like sparks change the position of their ends so rapidly that the 

 integrating action of the photographic film recorded a perfectly uniform 

 negative for exposure of fifteen seconds or more. The electrodes had to be 

 kept sharp and smooth, since when this was neglected the elementary sparks 

 persisted much longer in one position than in another, and, consequently, 

 caused streaks of varying intensity to run along the negatives parallel to 

 their length. 



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

 alternating E. M. F. of about 106 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 Leyden jar about 

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

 duced. 



EXPOSURES AND SPECTROGRAMS. 



When the complete spectrograms of a series of solutions were to be obtained 

 the succession of operations was in general as follows: 



First. The depth of the cell for plane parallel layers of solution was made 

 such that judging from experience, and being guided by the depth of the 

 color of the extreme solutions in their bottles, it would be neither too shal- 

 low to give appreciable absorption when the cell contained the most dilute 

 solution, nor too deep to transmit enough light to affect the photographic 

 plate when the liquid of deepest color was placed in the cell. 



Second. A portion of each of the extreme solutions was successively poured 

 into the cell which was, of course, clean and dry, and the corresponding 

 spectrum observed by the aid of the spectrograph and a suitable eyepiece. 

 In all cases, except those for which the solutions of deepest color were prac- 

 tically opaque to light, for all depths greater than one-half a millimeter, 

 a depth of cell was soon found which enabled the absorption spectra of all 

 the solutions of the series to be photographed on the same film, and with 

 exposures of equal duration. 



When fifteen or sixteen solutions belonged to one series, and this was 

 often the case, the first eight solutions were photographed on one-half of 

 a complete 5" X 7" negative film, and then the remaining solutions* had their 

 absorption spectra photographically recorded on the other half of the same 

 film. This break in the continuity of the work was due to the fact that 



* No series comprised more than sixteen solutions, 



