4:66 Wells and Penjreld — Thallium Triiodide, etc. 



caesium pentaiodide, but by the use of increasing proportions 

 of iodine with, thallium triiodide in alcoholic solutions no evi- 

 dence of the existence of such a compound could be obtained. 



The remarkably close relations of thallium to the alkali- 

 metals as far as the thallous compounds are concerned, and 

 the additional resemblance which has been pointed out in the 

 present communication, have led us to consider the possibility 

 that thallium has been wrongly placed in the periodic system 

 of the elements and that it really belongs to the alkali-metals. 

 There are two vacancies in Mendeleeff's table in the alkali- 

 metal group corresponding to atomic weights of about 170 and 

 220. One of these is smaller, the other larger than the 

 accepted atomic weight of thallium, so that, as far as these 

 numbers are concerned thallium might be composed of two 

 alkali-metal elements. Although the probability that thallium 

 was composed of two elements seemed very slight from other 

 considerations, we have deemed it desirable to test the ques- 

 tion experimentally. 



About 200 grms. of thallium were converted into the nitrate 

 and this was systematically fractionated by crystallization until 

 about one-twentieth of the salt remained as a repeatedly re- 

 crystallized portion and about another twentieth was contained 

 in a final mother-liquor. From each of these two fractions 

 thallous chloride was prepared by converting into sulphate, 

 precipitating impurities with hj T drogen sulphide, and finally 

 precipitating thallous chloride by means of hydrochloric acid. 

 The preparations were carefully washed, dried at 100°, and 

 the chlorine was determined as silver chloride in order to get 

 the atomic weight of the metal in each fraction. The silver 

 chloride was weighed in the Gooch crucible, a method which 

 can be most highly recommended for accurately weighing this 

 substance. The following results were obtained, the weights 

 being given as taken in air. 



Crystallized End. Soluble End. 



T1C1 taken 3-9146 g. 3-3415 g. 



AgCl obtaiued 2-3393 1-9968 



Atomic weight of TI, (0 = 16), 204-5 204-5 



It was not expected that absolute accuracy in the atomic 

 weight of thallium would be attained, but since the same 

 method of purification and analysis was used in both cases the 

 two results are comparable with each other, and their exact 

 agreement shows that the fractionation of the nitrate gives no 

 change in the atomic weight of thallium, and no evidence has 

 been obtained that thallium is not homogeneous. 



Sheffield Scientific School, New Haven, Conn., January, 1894. 



