both with selenium and sulphur. The research of Pellini and Vio 
also does not introduce any further complications; the two elements 
are united there in all proportions to one trigonal series, so that 
only the sulphur-containing complexes of selenium and tellurium 
exhibit the hiati on mixing in the solid condition. 
All this admits of the conclusion that the elements sulphur, selenium 
and tellurium form indeed a natural triad of perfectly homologous 
elements which are more adjacent to each other than their group- 
fellow oxygen is to any one of them. There can be only question 
of true “compounds” when one of the elements combines with oxygen *). 
$ 9. Now there is still the question: what must be thought of 
the tellurium-sulphur complexes which are formed, at the temperature 
of the room, by means of H 2 S from solutions of tellurites and 
tellurates, and in what sense must the so-called double sulphides 
obtained by Oppenheim and Berzelius be regarded. 
First of all, I soon succeeded in showing that the element tellurium 
and particularly its amorphous modification dissolves, without leaving 
any residue, when heated with a solution of alkali- or ammonium- 
sulphide, also that the solubility increases with the concentration of 
the sulphide; and further that the solubility also increases when 
potassium hydroxide is added to the sulphide solution, thus retarding 
the hydrolysis. Clear yellow solutions are so formed turning some¬ 
what ruddy on boiling, and oxidising rapidly in contact with the air 
with formation of a black precipitate. They are strongly alkaline and 
give with hydrochloric acid a heavy, black precipitate with evolution 
of HjS; the precipitate appears to contain tellurium as well as sulphur 
and is soluble in alkalihydroxyde. 
The analysis of these black precipitates did not give constant 
values; the tellurium content is much dependent on the modus operandi 
and oscillated between 46.9 % and 80.9 %. 
Thereupon, the action of H ? S on different tellurium compounds 
was investigated: on the basic nitrate, on the finely divided dioxide 
suspended in absolute alcohol, on dioxide in hydrochloric acid solution, 
on telluric acid in water and on the tellurite- and tellurate-solutions 
obtained from TeO, or telluric acid. A beautiful, somewhat crystalline 
looking product was obtained from the alcoholic suspension of TeO t ; 
the analysis of the blue-black substance gave 80.1 %—80.9 % of 
tellurium whereas theory requires 79.9% for TeS, 66.6% for TeS, 
eover, also known that 0 and S, for instance, never give isomorphous 
i organic compounds. S, Se and Te, however, behave differently as 
shown by the research of Pellini, Nobris, Tutton, and others. 
Proceedings Royal Acad. Amsterdam. Vol. XII. 
41 
