1864.] 



produced by Heat in Mineral Substances, 



241 



Magnus. I found, Jirst, that what lie stated for idocrase and for a speci- 

 men of reddish -brown garnet was also the case with the whole family of 

 garnets as well as the minerals of the idocrase group ; secondly ^ that it is 

 not necessary 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 

 month 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 a black enamel, accompanied with 

 very little leucite and traces of grossular, and crystallized in the second 

 system. 



Specimens weighing some grammes had their specific gravity taken with 

 great care, and by the method described by me in the * Chemical News ' for 

 1862. 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 trace of fusion, it 

 was allowed to cool, and when it had arrived at the temperature of the 

 atmosphere, its specific gravity was again taken by the same method as be- 

 fore. 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 increased in density 

 and had attained its former specific gravity. Thus : — 



Lime garnet {from Vesuvius). 



Density after being heated Density determined in 



red-hot for a quarter of an a month after the 



Original density. hour and allowed to cool. experiments. 



I. 3-345 2-978 3*344 



II. 3-350 2-980 3-350 



III. 3-349 2-977 .. , 3-345 



The same experiments were made with several other minerals belonging 

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

 I ask, what becomes of the heat that seems to be thus shut up in a mineral 

 substance for the space of a month ? The substance of the mineral is di- 

 lated, the distance between its molecules is enlarged, but these molecules 

 slowly approach each other again, and in the course of some weeks resume 

 their original positions. What induces the change ? or how does it happen 

 that the original specific gravity is not acquired immediately the substance 

 has cooled ? * Will the same phenomenon show itself with other famihes 

 of minerals or with the metalHc elements ? 



• Soma minerals , like euclase, that become electric by heat, retain that state for a 



