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POPULAR SCIENCE REVIEW. 
(so named after the celebrated Swedish mineralogist, Gahn) shows a larger 
percentage of zinc than any specimen of this mineral heretofore analysed ; 
it is associated with black mica, apatite, calcite, and a brownish variety of 
chrysolite, which, on partial analysis, was proved to he a mono- silicate of 
iron, manganese, and zinc. 
The Zircons of Mudgee, New South Wales.— Professor S. H. Church, M.A., 
gives an account of these in a recent number of the “ Chemical News.” 
He says he lately obtained a few rounded pebbles, each weighing about 
2 grammes, for examination; they came from Mudgee, New South Wales. 
Although the specimens did not present the usual lustre of worn surfaces of 
zircon pebbles, yet their obviously high density, and the traces they re- 
tained of their pyramidal form, nearly sufficed to identify them with 
the zircon ; this idea was amply confirmed by the results of experiment. 
The Mudgee zircons present the exact tint known as hyacinthine ; indeed 
the true hyacinth or jacinth, about which such constant mistakes are made 
by jewellers, lapidaries, gem-collectors, and even mineralogists, is now an 
attainable luxury. The engraved gems and the cut stones commonly called 
jacinths (even by Dana — vide his u Mineralogy,” fifth ed., p. 275), are in- 
variably, so far as his experience goes, nothing but the hyacinthine garnet, 
a comparatively common stone possessed of far less interesting properties 
than the true hyacinth. The Mudgee zircons are rather dark; the colour 
is distributed somewhat irregularly in and upon many of the specimens. 
When cut, facetted, and polished, this Australian zircon, if not too deep in 
colour and too large in size, presents a rich soft red colour tinctured with 
orange-brown. He was fortunate enough to secure one pale-coloured speci- 
men, which, owing to its having been judiciously cut, has turned out a 
stone of surpassing brilliancy and beauty. The density of one of the Mudgee 
stones was 4-704. After heating, it was found to have become quite colour- 
less, although its density remained virtually unchanged, namely, 4-699. In 
these and most other particulars the Mudgee specimens resemble those of 
similar colour from Expailly, in Auvergne. 
The Influence of Cold on Iron and Steel Railway Wheels. — This subject 
has been very largely taken up this year at Manchester, and the “ Proceedings 
of the Literary and Philosophic Society” have been filled with papers which 
are some of them exact contradictions of others. The following observations 
were made by Mr. Peter Spence at the meeting held March 10, 1871. 
After detailing some experiments, he says, his assistant then prepared a 
refrigerating mixture which stood at zero, and the bars were immersed for 
some time in this, and they prepared for the breaking trials to be made as 
quickly as could be, consistently with accuracy; and to secure the low tem- 
perature, each bar on being placed in the machine had its surface at top 
covered with the freezing mixture. The bars at zero broke with more 
regularity than at 60°; but, instead of the results confirming the general im- 
pression as to cold rendering iron more brittle, they are calculated to sub- 
stantiate an exactly opposite idea, namely, that reduction of temperature, 
cveteris paribus, increases the strength of cast-iron. The only doubtful 
experiment of the whole twelve is the first, and as it stands much the 
highest, the probability is that it should be lower ; yet, even taking it as it 
stands, the average of the six experiments at 60° F., gives 4 cwts. 4 lbs. a3 
