1863.] 409 



paper communicated to the Royal Society in June last. An impres- 

 sion extending to about division 154 of the scale then adopted was 

 obtained. This spectrum contains several very characteristic groups 

 of lines ; it recalls the features of the spectra of cadmium and zinc, 

 and less strongly that of lead. 



Measuring by the scale already adopted in my former paper, it is 

 found that there are two strong groups of lines at about 103 and 

 106. At 116, 121, and 126 are three groups the first two less 

 intense than the third, which is of about the same strength as the 

 earliest two. Several feebler pairs of dots follow, and the spectrum 

 terminates rather abruptly with four nearly equidistant groups, com- 

 mencing respectively at 136, 141, 145, and 151. The first of these 

 groups is very strongly marked, the others are fainter, but of nearly 

 equal intensity. 



The remarkable way in which a spectrum at low temperatures so 

 simple becomes increased in complexity, both in the visible and in 

 the extra-visible portions, is of high interest considered in relation 

 to the physical cause of these phenomena; and it is not without 

 interest in a chemical sense, from its bearing upon the view sup- 

 ported by Dumas, that thallium belongs to the alkaline group. 

 Potassium and sodium exhibit no new lines in the induction-spark, 

 merely a diffuse light filling up the air-lines, and lithium but a 

 single strong group at about 124. This physical character, added 

 to the more purely chemical ones of the insolubility of the sulphide, 

 the chromate, the iodide, the sparing solubility of the chloride, the 

 phosphate, the oxalate, the ferrocyanide, the occurrence of a power- 

 fully basic oxide, and of a higher feebly acid oxide, may therefore 

 assist in showing the resemblance of thallium to silver or to lead, 

 which latter metal in density, colour, softness, and external appear- 

 ance it so closely simulates. 



It would be easy to point out other particulars in which the proper- 

 ties of thallium are in strong contrast with those of the alkali metals. 

 The chemical energy of these metals, lithium, sodium, potassium, 

 rubidium, and caesium, increases in the order mentioned, which is 

 that of their equivalents. Thallium, with a higher equivalent than 

 any of these, shows a greatly diminished chemical activity. The metal 

 is readily reduced by zinc from its solutions. Its oxide, instead of 

 being like that of all the alkalies, excessively deliquescent, is perma- 



