(610 ) 
rich in sulphur appear to change into the rhombic form at a lower 
temperature. In these circumstances nothing is noticed as to compounds 
between tellurium and sulphur; even at lower temperatures no heat 
effects are observed. The melts of the mixtures rich in tellurium 
are dark brownish black and in thin layers yellowish brown; unlike 
the melts rich in sulphur they are thin fluid up to their solidifying 
points. 
§ 8. Considering all that is known up to the present as to the 
behaviour of the elements sulphur, selenium, and tellurium on being 
melted together, we may say that in this respect, tellurium certainly 
deserves the place assigned to it by Dumas. Sulphur and selenium 
form, according to Ringer 1 ), a trimorphons series of mixed crystals, 
selenium and tellurium, according to Pellini and Vio *) an uninter¬ 
rupted series of trigonal mixed crystals; but no compounds are formed, 
as may be expected, looking at the experience gained, apart 
from the exceptions in such triads of homologous elements, at 
any rate in the central groups of the periodic system. With sulphur 
and selenium the matter is even somewhat, still more complicated, 
as three instead of two heteromorphous kinds of mixed crystals 
occur in this case. If we accept Ret'gers’ view according to whom 
a less stable form, mostly unknown in the free state, of each of 
the components should correspond to each of these forms, the isotri¬ 
morphism in the case of selenium and sulphur is certainly more 
difficult to explain than the dimorphism of sulphur and tellurium. 
For of the two monoclinic series in the system: sulphur-selenium 
one, according to Muthmann, is analogous to the form of y-sulphur, 
whereas the trigonal series would already possess the form of metallic 
selenium. But neither of the two known monoclinic modifications of 
selenium is isomorphous with any monoclinic modification of sulphur s ), 
whilst the trigonal so-called t-form of this element differs from the 
trigonal form of selenium. Looking from Retgers’ standpoint both 
these elements should be credited, in addition to their well known 
allotropic forms, with at least another two unknown, less stable 
modifications. In the trigonal series of the system : sulphur-tellurium 
we are dealing obviously with the same less stable trigonal form 
of sulphur as in Ringer’s investigation, whilst the assumption of an 
uustable monoclinic form of tellurium cannot have anything artificial 
about it, in view of the fact that this symmetry occurs frequently 
l ) Ringer, Z. f. anorg. Chem 32. 183. (1902). 
-) Pellini and Vio, Gazz. China. It. (1906). II. 476. 
*) Groth, Ghemische Krystallographie, Bd. 1. (1906). p. 23—35. 
