370 Mr. H. Wilde. On the Spectrum of Thallium, [Apr. 20, 



The mouths of the phials were closed with tightly-fitting stoppers 

 of caoutchouc, through which pairs of copper wires were thrust for 

 the purpose of attaching electrodes of different metals in the in- 

 terior of the phials. Pairs of platinum and thallium electrodes were 

 placed in the phials respectively, which were set aside for some 

 hours to allow the moisture of the enclosed air to be absorbed. 



An induction coil, giving a 10-inch spark in air with 12 amperes 

 of current, was used in the experiments, and the density of the spark 

 was increased by connecting the secondary circuit with the coatings 

 of a Leyden jar in the usual manner. 



The observations were made with a direct-vision spectroscope of 

 five prisms, fitted with an illuminated scale, which enabled compari- 

 sons to be made simultaneously between the spectral lines of different 

 substances with great exactness and rapidity. The range of the in- 

 strument with the arc spectrum included the rubidium line 7951, and 

 the calcium line H 2 3933, while the prismatic dispersion was such as 

 to well divide the potassium double line 4044, 4042. 



On transmitting a succession of sparks from the platinum electrodes 

 through the dried air in one of the phials, the air lines 6602 and 

 6482 were very conspicuous, but there was no trace of the hydrogen 

 line 6562 between them. When, however, the sparks were taken 

 from the thallium electrodes under like conditions the sharp red line 

 appeared as in the arc spectrum. The same results were obtained 

 when similar pairs of platinum and thallium electrodes were con- 

 nected in series and placed under a large glass receiver, the air in 

 which had been dried by sulphuric acid. 



The red line of thallium does not appear in the oxy-hydrogen flame, 

 nor when the intensity of the spark is somewhat reduced by placing 

 a vacuum tube in series with the thallium electrodes, although the 

 brilliancy of the air lines was but little diminished. 



That the red line was not due to hydrogen occluded in the thallium 

 was shown by the following experiments : — A pair of palladium 

 electrodes were saturated with electrolytic hydrogen from dilute 

 sulphuric acid, through which an electric current was transmitted for 

 several minutes. These electrodes were substituted for those of 

 platinum in the phial of dried air ; and when the sparks passed 

 between them the strong hydrogen line C appeared for several 

 seconds before it finally disappeared through the heating of the 

 pa'ladium electrodes. 



A small quantity of thallium was placed on a piece of pumice under 

 the glass receiver of an air pump, and fused in vacuo by the arc 

 light condensed by a pair of Fresnel annular lenses focussed through 

 the side of the receiver. The spectrum of the thallium fused in vacuo, 

 from which all gas had been excluded, was the same as in the pre- 

 vious experimen.s. 



