SECT. iv. SPECTR UM OF INDIUM. 1 3 7 



flash, lasting but the fraction of a second. If a larger 

 quantity of the metal be gradually, put into the flame, 

 it lasts a little longer, appearing as a single green line 

 of extraordinary purity and intensity, sharply denned 

 on a black ground. With respect to volatility, thallium 

 is analogous to the non-metallic element selenium, 

 which is so volatile that its beautiful blue light only 

 lasts a few seconds. The green light of thallium 

 comes out more rapidly, and with less of the substance, 

 than the blue light of selenium, a quantitative dis- 

 tinction which accords with Dr. Miller's observation 

 that the rapidity with which a result is obtained, and 

 the minuteness of the quantity required for the exami- 

 nation, gives this method a superiority over every other 

 for the qualitative analysis of the alkalies and alkaline 

 earths. Thallium has been detected in mineral waters, 

 wine, treacle, tobacco, and chicory. 



Drs. Reich and Richter discovered a fourth new metal 

 in the zinc-blende at Freiberg in Saxony, which has 

 been called indium, from two beautiful indigo-blue lines 

 in its spectrum, which have a greater refrangibility than 

 the blue lines in strontium. The chemical relations of 

 indium resemble those of zinc, with which it is associated 

 in nature. The metal can be reduced before the blowpipe 

 into a bead, which marks paper and has the colour of tin. 



The practical importance of spectrum science has been i 

 beautifully illustrated by Professor Roscoe by its applica-4 

 tion to overcome a difiiculty in Bessemer' s process for the: 

 manufacture of steel. According to that process, steel 

 is made by sending a blast of air through a quantity of 

 melted iron ; the difficulty was when to stop the blast, 

 for if stopped too soon, the metal retains so much carbon 

 that it crumbles under the hammer ; if continued a few 

 minutes too long, the molten metal is so viscid that it 

 cannot be poured into the moulds. Experience had 

 hitherto enabled the manufacturer to judge of the 



