298 



Sir W. Crookes. 



[Aug. 1, 



film, overlapping for about 1 mm. in the centre.* The photographs 

 are then transferred to a measuring machine, and from the figures 

 thus obtained the wave-lengths are calculated. 



The radium was used in the form of nitrate, a well-crystallised salt 

 easily soluble in water. The induction spark was taken between 

 platinum poles partly immersed in a strong solution of the salt. On 

 this account the nitrate was used in preference to the chloride. Much 

 platinum is always dissolved when the poles are sparked in a solution 

 of a chloride, while with nitrates this does not occur to the same 

 extent. 



The solution was strong, and slightly acid with nitric acid. Many 

 forms of spark tube were tried before a satisfactory form (shown in 

 fig. 2) was devised. AB is a tube of hard Jena glass, with a bulb at 



FIG. 2. 



the lower end. At the bottom of the bulb a platinum wire is sealed 

 in, and in the upper part of the tube, at B, a short piece of tube is 

 ground in to form a stopper. A long platinum wire passes through 

 from this stopper, bare at the upper part, and guarded with glass 



* ' Key. Soc. Proc.,' vol. 65, p. 243. 



