NATURAL PHILOSOPHY. 1G1 



NOTES ON THE VARIATIONS OF DENSITY PRODUCED BY HEAT 

 IN MINERAL SUBSTANCES. BY DR. T. L. PHIPSON, F- C. 8. 



" That any mineral substance, whether crystallized or not, should 

 diminish in density by the action of heat, might be looked upon as a 

 natural consequence of dilation being produced in every case and be- 

 come permanent. Such diminution of density occurs with idocrase, 

 Labradorite, feldspar, quartz, amphibole, pyroxene, peridote, Sam- 

 arshite, porcelain and glass. But Gadolinite, zircons, and yellow 

 obsidian augment in density from the same cause. This again may 

 be explained by assuming that, under the influence of a powerful heat, 

 these substances undergo some permanent molecular change. But in 

 this note I have to show that this molecular change is not permanent, 

 but intermittent, at least as regards the species 1 have examined, and 

 probably with all others. Such researches, while tending to elucidate 

 certain points of chemical geology, may likewise add something to 

 our present knowledge of the modes of action of heat. My exper- 

 iments were undertaken to prove an interesting fact announced 

 formerly by Magnus, namely, that specimens of idocrase after fusion 

 had diminished considerably in density without undergoing any change 

 of composition ; before fusion their specific gravity ranged from 3'349 

 to 3*45 and after fusion only 2*93 to 2'945. Having lately received 

 specimens of 'this and other minerals brought from Vesuvius, I deter- 

 mined upon repeating this experiment of Magnus. I found, first, 

 that what he stated for idocrase and for a specimen of reddish-brown 

 garnet was also the case with the whole family of garnets as well as 

 for the minerals of the idocrase group e : secondly, that it is not ne- 

 cessary to melt the minerals ; it is sufficient that they should be heated 

 to redness without fusion, in order to occasion this change of density : 

 thirdly, that the diminished density thus produced by the action of a 

 red heat is not a permanent state, but that the specimens, in the course 

 of a mouth or less, resume their original specific gravities. These 

 curious results were first obtained by me with a species of lime garnet 

 in small yellowish crystals, exceedingly brilliant and resinous, almost 

 granular, fusing with difficulty to black enamel, accompanied with 

 very little leucite and crystallized in the second system. 

 * * * * Specimens weighing some grammes had their specific 

 gravity taken with great care. They were then perfectly dried and 

 exposed for about a quarter of an hour to a bright red heat. When 

 the whole substance of the specimen was observed to have attained 

 this temperature, without a trace of fusion, it was allowed to cool, 

 and when it had arrived at the temperature of the atmosphere, its spe- 

 cific gravity was again taken by the same method as before. The 

 diminution of density being noted, the specimens were carefully dried, 

 enveloped in several folds of filtering paper, and put aside in a box 

 along with other minerals. In the course of a month it occurred to 

 me that it would be interesting to take the specific gravity again, in 

 order to ascertain whether it had not returned to its original figure, 

 when, to my surprise, I found that each specimen had effectively in- 

 creased in density and had attained its former specific gravity. The 

 same experiments were made with several other minerals belonging 

 to the idocrase and garnet family, and always with similar results. 

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