Mr. A. K. Tuttn. A >:/<,jrj>/< /'<<>/ 



depends ha\e been made by Mr. W. Hugo, senior assistant at I\c\\ 

 Observatory, to whose care and exactitude the research owes a very 

 great deal. The reduction of the observations is also mainly due to 

 Mr. Hugo. In several of the special experiments Dr. J. A Harker 

 haa given valuable assistance, and various operations have been much 

 facilitated by means of apparatus devised by him. The main work 

 of checking the reductions and the various calculations requisite 

 in discussing the observations have been done by myself. I have 

 also taken a considerable number of observations, and in discussing 

 the various sources of trouble I speak from personal experience, supple- 

 mented of course by frequent exchange of ideas with Dr. Harker and 

 Mr. Hugo. At various stages of the investigation I have had valuable 

 advice and assistance from Professor Carey Foster and Mr. W. N. Shaw, 

 to whom the initiation of the research is largely due. 



The original cost of the apparatus was partly defrayed by a grant of 

 100 from the Government Grant Fund, and a contribution of 120 

 was received last year from the Gunning Fund towards this and other 

 cognate work undertaken by the Kew Observatory Committee. 



" A Comparative Crystallographical Study of the Double Selenates 

 of the Series E 2 M(Se0 4 ) 2 ,6H 2 0. Salts in which M is Zinc." 

 By A. E. TUTTON, B.Sc^F.RS. Received March 5, Eead 

 March 15, 1900. 



(Extended Abstract.) 



In two communications to the Chemical Society,* the author 

 presented the results of a detailed study of twenty-two salts of 

 the series of monoclinic double sulphates R 2 M(S0 4 )o,6H.,0, in which 

 R was represented by potassium, rubidium, and caesium, and M by 

 magnesium, zinc, iron, manganese, nickel, cobalt, copper, and cadmium. 

 The first of these memoirs dealt with the external morphology of the 

 crystals, and the second with their internal physical properties. 



The present investigation refers to the less-known analogous double 

 selenates, in which R is again represented by the alkali metals potassium 

 (atomic weight 39), rubidium (atomic weight 85 -2), and caesium (atomic 

 weight 132'7). The work on the group containing zinc has been com- 

 pleted, and the results are now communicated. Topsoe and Christian- 

 sent included in their well-known investigation the potassium salt of 

 the group. 



* ' Journ. Chem. Soc. Trans.,' 1893, vol. 63, p. 337, and 1896, vol. 69, p. 344. 

 t ' Ann. de Chim. et de Phys.,' 1874, p. 5. 



