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POPULAR SCIENCE REVIEW. 
is dimorphous and may also crystallise in the triclinic system, hut this has 
not been universally admitted. The opinion that silica of low density is 
always amorphous is, said the author, erroneous : it exists in crystals. 
These crystals are colourless and limpid, with polished brilliant faces. 
Their measurement is difficult, because of their small size (hardly one milli- 
metre). They are arranged in groups of twos and threes, and the latter 
being the most common, the term Tridymite has been given to this new 
mineral. Its system is the hexagonal, but very different ^from that of 
quartz. The fundamental form is the hexagonal dodecahedron. The hard- 
ness is equal, or nearly so, to that of quartz, and separate crystals are very 
rare. The specific gravity, taken at from 15° to 16° C., is from 2-326 and 
2-312 to 2-295. Tridymite is infusible in the blowpipe, and its blowpipe 
reactions present no remarkable features. The analysis of the crystals 
(pounded in a steel mortar and fused with carbonate of soda) gave the 
following results in two cases : — 
1st 2nd 
Silicic acid 96-1 95-5 
Oxide of iron 1-9 1-7 
Magnesia and alumina , . .1-3 1-2 
Loss 0-66 0-66 
99-96 99^ 
The iron, the author thinks, was derived from the mortar, and the other 
elements from the matrix in which the crystals presented themselves. 
Alloys of Copper and Tin . — The Paris Correspondent of the Chemical 
News (whose letter, by the way, is always full of interesting details) states 
that in a note communicated to the Academy by M. Kiche, the following 
facts concerning the alloys of copper and tin are given. The question of 
density is first taken. Some determinations were made upon bars of these 
metals, weighing from 50 to 60 grms., but the results obtained were 
unimportant, owing to the great difference which exists in these alloys. 
The same metals, reduced to fine powder, were afterwards operated upon, 
when it was observed that the contraction increases very regularly from the 
very rich alloy in tin to the mixture SnCu 2 , and from this point it increases 
suddenly, arriving at a maximum, when the copper and tin are united in 
the relation of 1 to 3. The density diminishes from this point, then rises 
again nearly regulai’ly ; the density of the richest copper alloys is inferior to 
the mixture SnCug, which only contains 62 per cent, of copper. Besides,' 
this alloy may be distinguished from all the others by its properties ; it is 
brittle enough to be pounded in a mortar, and forms crystals of a bluish 
tint, not resembling in the least either copper or tin. M. Riche gives a 
number of formula), expressing the composition of the definite compounds 
which copper forms with tin and their properties. Refemng to liquefaction, 
he then observes : “In order to separate these alloys, the mass should be 
moved about wlien becoming solid, to separate the crystals whilst forming.” 
The fusibility of these alloys has been determined by the thermo-electric 
pyrometer. M. Itiche has operated comparatively with these alloys, and 
with metals whose fusing points have been settled by various experimenters. 
Numerous determinations show that the solidification of the alloys SnCuj 
and SnCu^ takes place at a temperature somewhere between the fusion point 
