468 Mr. H. Wilde on Helium and its place in the 



visible spectrum of thallium for more than thirty years, so 

 that the sharp red line in the arc- and spark-spectrum of this 

 element is not mapped in the recent tables of Thalen, and 

 Kayser and Runge *. 



I have recently repeated some of the experiments of 

 Ramsay and Lockyer on helium obtained by the distillation 

 method from Norwegian cleveite, pitchblende, and other 

 minerals containing uranium. The result of these experi- 

 ments confirms the conclusion that the differences in the 

 determination of the wave-lengths of the components of the 

 characteristic yellow line are due to the same cause which 

 masked the red line of thallium. 



The apparatus with which the experiments were made is 

 shown in Plate VIII. one-fourth the actual size. It consists 

 of a small steel cylinder heated from below by a Bunsen 

 burner, or the oxyhydrogen-flame. A bent iron tube, of 

 small bore, connects the cylinder with an air-pump and a 

 glass sparking-receiver in which the spectra of the gases are 

 produced. The mouth of the receiver is plugged with a 

 stopper of caoutchouc, through which a pair of iron wires 

 are thrust terminating with platinum electrodes. Vacuum- 

 gauges are mounted on the pump for measuring the amount 

 of rarefaction in the cylinder and sparking-receiver to a small 

 fraction of an inch of mercury. 



An induction-coil giving a 10-inch spark in air was used 

 in the experiments, and the density of the spark was increased 

 by means of a Leyden jar. The observations were made with 

 the same direct- vision spectroscope of five prisms as was used 

 in my research on the spectrum of thallium. 



The minerals, in coarse powder, from which the gases are 

 to be distilled are fed into the cylinder through the end into 

 which the tube is screwed, and the joint is afterwards made 

 good by means of a washer of asbestos. 



Dr. Burghardt, Lecturer on Mineralogy at the Owens 

 College, kindly placed at my disposal some heavy zirconi- 

 ferous sand, containing uranium, which is found in large 

 deposits on the coast of Brazil. This sand is an abundant 

 source of helium, and, judging from the brightness of the 

 spectroscopic reaction, is not much inferior to that of cleveite. 



14 grammes of the sand were fed into the cylinder, which, 

 after being exhausted of air,was heated up by the Bunsen flame. 

 As the heat, of the cylinder approaches visible redness, the 

 double sodium line and the C hydrogen line make their 

 appearance, and when the pressure in the receiver increases 

 to six inches of mercury the yellow line and the violet line 



* Ueberdie Spectrender Ekme7ite, pt. vi., Berlin, 1892. British Asso- 

 ciation Report, 1893, p. 403. 



