clarke.] ORTHOSILICATES OF DYAD BASES. 73 
upon by the reagent. Brauns's objections* to this supposition, on the 
ground that the dry hydrochloric acid becomes moist, are not well 
taken, for the reaction was always applied at temperatures lower than 
those at which water is given oft*. His criticisms may apply to the 
later stages of the reaction, after it has once fairly begun, but not to 
its initiation. The magnesian micas which contain several per cent of 
water are all decomposable by aqueous hydrochloric acid, but are 
scarcely touched by the dry gas, while on the other hand serpentine 
and the chlorites are strongly attacked. After the gaseous acid has 
acted it becomes moist, but very slowly, and most of the moisture is 
carried past the mineral under investigation before it has had time to 
produce an appreciable effect. It is possible, however, that a slow 
stream of the acid may act differently from a rapid current, and that 
the discordant results of observation may be due to differences of this 
kind. 
When serpentine is ignited, water is expelled, and a residue having 
the composition Mg 3 Si 2 7 is left behind. According to Eammelsberg t 
the water is given off in two portions — one-half upon weak ignition, the 
other after heating more strongly. On the orthosilicate theory these 
stages may be represented thus : 
Serpentine. First stage. Second stage. 
H 2 
II /Si0 3x /SiO-K 
Si0 4X Mg< >Mg Mg< >Mg 
Mg/ 4 \vig \si0 4 / ^SiO, 
> i0 V h/ \m 8 -oh ii 
H/ ^Mg— OH 
Mi 
At the end of the second stage, if the ignition has not been too intense, 
the residue is still decomposable by hydrochloric acid, but by prolouged 
heating it is broken up quantitatively into soluble olivine and insoluble 
enstatite. 
Allied to serpentine is the somewhat doubtful pierosmine, to which 
the formula Mg 2 H 2 Si 2 7 is commonly assigned. Although this expres- 
sion suggests a diorthosilicate, it may also be written 
/Si0 3x 
Mg/ ^ \Mg 
II 4 
H 2 
which represents pierosmine as a dehydrated serpentine altered sub- 
sequently by rehydration, with replacement of one magnesium atom by 
two of hydrogen. This mode of interpretation brings the mineral into 
line with serpentine, and all the known relations of the species are 
adequately expressed 
* STeues Jahrbuoh, 1894. 1, p. 205. 
t Handbuch der Minei alchemic, 2 Aufl., p. 506. 
