1862.] 157 



point. When a minute portion of the metal (the sulphide, chloride, 

 or, in fact, any compound of thallium) is introduced into the flame of 

 the spectroscope, it immediately produces a single green line, per- 

 fectly sharp and well defined upon a black ground, and of extraor- 

 dinary purity and intensity, almost equal to the sodium-line in bril- 

 liancy. It is not, however, very lasting. Owing to its great volatility, 

 a portion introduced at once into the flame merely shows the line as 

 a brilliant flash, remaining only a fraction of a second ; but if it be 

 introduced into the flame gradually, the line continues present for a 

 much longer time. If, also, a piece of metallic thallium be intro- 

 duced into the flame on a platinum wire loop, they fuse together, and 

 the alloy gives the green line rather more permanently, although of 

 course fainter. 



Working on a small scale, it is not easy to obtain these compounds 

 free from soda ; but when that is effected, and a tolerable quantity of 

 substance is held on a loop of platinum wire in a flame, the green 

 colour is most brilliant, and produces very extraordinary effects upon 

 the appearance of surrounding objects. If thallium could be 

 obtained in quantity, this ready means of producing an intense and 

 homogeneous green light could not fail to be applicable to some useful 

 purpose. 



The green line of the thallium spectrum appears to be unaccom- 

 panied by any line or band in other parts of the spectrum. A flame 

 of sufficient temperature to bring the orange line of lithium into view 

 produces no addition to the one thallium -line ; and an application of 

 telescopic power strong enough to separate the two sodium-lines a 

 considerable distance apart still shows the thallium-line single. I 

 consider therefore that I am justified in stating that thallium pro- 

 duces the simplest spectrum of any known element. Theoretical 

 inquiries into the cause of the spectrum lines, and their relation to 

 other constants of an element, may be facilitated now we know a metal 

 which gives rise to luminous vibrations of only one degree of refran- 

 gibility. The remarkable simplicity of the thallium spectrum offers 

 a strong contrast to the complicated spectra given by mercury, 

 bismuth, and lead the metals to which it has the most chemical 

 resemblance. 



The position of the green line does not coincide with any definite 

 line in the solar spectrum. According to KirchhofFs theory, we must 



