86 Transactions of the 



other two lines never change their position, whatever may be the 

 temperature to which the vapour is raised. Whenever substances 

 are mixed together, the spectrum of neither is spoiled, but the 

 most volatile body gives the first appearance of its lines. 



Potassium, when placed in the flame, gives two characteristic 

 lines, namely, one line, K a, in the outermost, red, approaching the 

 ultra red rays, exactly coinciding with the dark line A of the solar 

 spectrum ; and a second line, K yS, situated in the extreme violet 

 rays, and also identical with a particular dark line observed by 

 Fraunhofer. There is also another very indistinct line, coinciding 

 with the dark line B, but this is seen only when the light is intense, 

 and is by no means characteristic. In order to detect the presence 

 of potassium, the eye requires j^ of a milligramme of potassium 

 (chlorate). 



Btrontvum has a much more complicated spectrum than any 

 of the former substances ; in fact, all the alkaline earths have. 

 In it there are eight lines remarkable, six red, one orange, 

 and one blue. There are no green bands present. ^^-^^^ of a 

 milligramme, or j;^,,^;^ of a grain, can be with certainty detected. 

 Strontia has been shown to exist in very many limestones of 

 different geological ages. 



The spectrum of calcium is particularly marked by the very 

 bright green line Ca /S. There is also a very bright orange line 

 Ca a, nearer the red end of the spectrum than either the sodium 

 line, Na a, or the orange band of strontium, S?' a. There are 

 several other lines, but they are not of so much importance as the 

 other two lines. In order to see these lines, the eye requires just 

 as much calcium as strontium. 



Of all the alkalies and alkaline earths, barium has the most 

 complicated spectrum. It can easily be distinguished by the 

 green lines Ba a and Bo. /S. These are by far the most distinct. 

 Ba 7, although not so distinct, is nevertheless a well-marked and 

 peculiar line. About j^j of a milligramme of a barium salt may 

 be detected with tlie greatest certainty. 



The spectra of caesium, rubidium, and potassium present a 

 striking analogy in their appearance. They each possess a 

 spectrum which is continuous in the middle, showing that a 

 substance could give out rays of every degree of refrangibility. 

 Rubidium shows two splendid blue lines, two beautiful red lines, 

 besides several in the green and orange. Caesium gives two char- 

 acteristic blue lines, which are not so near the violet end as those 

 of rubidium. Not long after the discovery of the two new 

 alkalies, Mr Crookes, in examining the deposit of the flue of a 

 pyrites burner, found a new substance, which he called Thallium, 

 owing to the splendid solitary green line which it gives. It is 

 not difficult to obtain the metal from its salts. 



