176 ANNUAL OF SCIENTIFIC DISCOVERY. 



Tilkerode. Many of the chemical properties of this substance induced 

 Lini to believe that the rare metal tellurium was present ; but all at- 

 tempts at a separation of this metal by chemical means failed. Still 

 unsatisfied of its absence, he then had recourse to the most recent 

 method of chemical analysis by means of the spectroscope. In the 

 spectrum obtained, there was no evidence of tellurium, but a magnifi- 

 cent green band was observed, which had never been noticed in the 

 spectrum of any known element. It was this that convinced Mr. 

 Crookes that he had in the substance under examination a body en- 

 dowed with properties distinct from those of any other, and in course 

 of time led him to the isolation of the new metal. He pointed out a 

 curious parallelism between tho history of the discovery of thallium and 

 of selenium. The great Swedish chemist Berzelius, a little more than 

 half a century ago, was engaged on the examination of a residue from 

 a sulphuric acid manufactory, similar to that Mr. Crookes examined. 

 Berzelius, too, was looking for tellurium. He failed to find it ; but in 

 the course of his examination he discovered selenium, a body belonging 

 to the sulphur group of elements. Had Berzelius been acquainted 

 with spectrum analysis, Mr. Crookes remarked, it is very probable he 

 would have discovered thallium also, since the new metal gives the 

 simplest and best-defined spectrum hitherto observed. Mr. Crookes 

 first succeeded in isolating thallium by the use of voltaic electricity in 

 September, 1861. It is precipitated from its solutions by this means, 

 like lead and tin ; but, if the precipitation be carefully conducted, the 

 metal crystallizes out in elegant tufts, which spread out in branches re- 

 sembling some of the delicate seaweeds. This was beautifully shown 

 on a screen by means of the electric light. When precipitated more 

 rapidly, it comes down in a spongy state, and can then be easily pressed 

 into an ingot. 



Report of the French Academy on Thallium. A committee of the 

 French Academy, instructed to report on the subject of the new metal, 

 in a paper presented to them during the past year, remarks, " That 

 its discovery forms an epoch in the history of chemistry, on account of 

 the astonishing contrast between its chemical and physical characters 

 and they call it the " ornithorynchus of metals." It has nearly the 

 same appearance as lead, may be cut in a similar manner, and leaves 

 a similar trace on paper. It has, moreover, the same density, nearly 

 the same fusing point, and the same specific heat. Its solutions, like 

 those of lead, yield a black precipitate with sulphuretted hydrogen, a 

 yellow one with iodides and chromates, and a white one with chlorides. 

 But it indubitably belongs to the family of alkaline metals, which re- 

 cent discoveries have doubled in number. In this list thallium stands, 

 as regards the weight of its equivalent, at the opposite extremity of the 

 scale to lithium, the numbers being, lithium, 7 ; sodium, 23 ; potas- 

 sium, 39 ; rubidium, 85 ; coesimn, 1 20 ; thallium, 204. The commis- 

 sion remark that the equivalent of sodium is exactly the mean between 

 that of potassium and lithium ; that if double the atomic weight of 

 sodium is added to that of potassium, the equivalent of rubidium is 

 obtained ; that adding double the weight of sodium to double that of 

 potassium gives very nearly that of coesium ; and that adding double 

 the weight of sodium to four times the weight of potassium, gives 

 nearly that of thallium. The equivalents of all the alkaline metals, 



