120 Dr. S. P. Thompson on the Use of 



If it is an earth which is required, why not endeavour to 

 fractionate alumina, an earth which, though resembling other 

 earths of its class, to a certain extent is yet in many of its 

 properties totally distinct from them ? Again, if it is thought 

 possible, were suitable means known to fractionate any ele- 

 ment, why not endeavour to do so in the case of iodine ? 

 This element, being situated at the extreme end of its series, 

 differs as much as it could possibly do from caesium, the 

 element next to it in order of atomic weights. If, therefore, 

 it were possible to separate iodine into bodies with atomic 

 weights approaching gradually to that of caesium, might it 

 not be thought that a very slight difference of atomic weight 

 would correspond to a very much larger variation in chemical 

 properties ? 



The actual fact of there being such a large number of 

 these rare earths so very similar in properties is to be partly 

 explained by the fact that not only these elements, but also 

 the other members composing the table, show extreme varia- 

 tions among themselves to a smaller and smaller extent as 

 their atomic weights get larger and larger. Also all these 

 elements, except samarium and mosandrium, find their places 

 in the third, fourth, and fifth groups, and are on this account 

 naturally to a certain extent similar in properties. 



It is hoped that in this paper the views and actual results 

 of any chemist have not been misinterpreted. It has been 

 endeavoured as far as possible to be correct, but if any slight 

 errors have been committed, it is hoped that they will be 

 recognized as being unintentional. 



The University Laboratory, 

 Oxford. 



XIV. On the Use of Fluor- Spar in Optical Instruments. 

 By Professor Silvanus P. Thompson, D.Sc, fyc* 



CONSIDERING the very distinctive optical properties of 

 fluor-spar, it is remarkable that hitherto so little use has 

 been made of it in the optical industries. Its low refractive 

 power, and its still lower relative dispersion, make it remark- 

 able amongst transparent media ; the only substances com- 

 parable with it in these respects being rock-salt and rock- 

 alum, both of which being soluble in water are for that reason 



* Communicated by the Author, having been read before Section A of 

 the British Association at Leeds. 



