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SCIENTIFIC NEWS. 



[Aug. 10, 18 



for the diffusion of science — the immortal Faraday has 

 expounded with such generous ardour Ebelmen's 

 wonderful achievements in mineralogical synthesis. 



We have already indicated the data furnished by 

 chemical and mineralogical analysis, which these 

 scientists had as foundation for their attempts. One 

 point which we have not yet touched upon, is the 

 base of their general process. Theory would suggest that 

 the most ancient crystals of an igneous rock should be 

 the least fusible, and observation corroborates this theory, 

 the minerals of the first period of crystallisation occupy- 

 ing the lowest place in the scale of fusibility. The con- 

 stituent specimens of the lavas have appeared at succes- 

 sive times, according to their degree of fusibility, in 

 proportions as the temperature fell. These facts, ascer- 

 tained in detail by microscopic analysis, served as the 

 point of departure for MM. Fouque' and Levy's labours. 

 Their process rests also partly on a fact which Sir James 

 Hall had foreseen, viz., that the fusion of a rock pro- 

 duces an artificial rock more easily fusible than are any 

 of its crystalline constituents. Now, if a natural rock be 

 submitted to a very high temperature, and the glass pro- 

 duced by this fusion made to pass through a series of 

 decreasing temperatures, always, however, higher than 

 that of the melting point of the vitreous mass, the 

 minerals which can crystallise in the magma are formed 

 in due succession, the least fusible separating first. These 

 crystals are eventually united and surrounded by those 

 whose fusibility is greater, which appear in their turn 

 as the temperature decreases. Without entering 

 into the technical details of the apparatus, we will 

 content ourselves with remarking that by the aid 

 of the furnaces and tubes which MM. Fouque and Levy 

 used for their synthesis, all intermediate stages, from 

 glowing red to dazzling white heat, can be obtained, and 

 a given temperature maintained undeviatingly duiing an 

 unlimited period. Into a furnace is introduced a 

 platinum crucible, of a capacity of about twenty cubic 

 centimetres, containing the mixture of mineral matters 

 which fusion, and subsequent annealing, are to trans- 

 form into rocks. The following is the sequence of the 

 operation : first, by the aid of special arrangements the 

 temperature is kept at dazzling white heat, while the mix- 

 ture is transformed into glass. By regulating the admission 

 of gas and air, and by uncovering the furnace, the tem- 

 perature of the molten mass is reduced to orange-red 

 heat or about the melting point of steel. The crucible is 

 then raised from the furnace, when the temperature de- 

 creases to cherry-red, or the fusion point of copper. 

 Finally, if the crucible be taken quite out of the kiln, 

 its contents can still be maintained at a temperature at 

 which copper would only melt with difficulty. 



We have sketched the broad outlines of the operation. 

 It is practically a succession of annealings at diminishing 

 temperatures, which force the crystals to form in a series 

 commencing with the least fusible, and which give the 

 texture and mineralogical composition of volcanic pro- 

 ducts to the molten matter subjected to this treatment. 



We will explain, by a few examples, the mode of 

 operation for lithological syntheses. Let us follow the 

 method of reproduction of one of the rocks, which takes 

 a prominent part in the eruption of Vesuvius, leucote- 

 phrite, a rock composed of leucite, labradorite, and 

 augite. 



A mixture is formed of silica, alumina, lime, ferrous 

 oxide, potassium, and soda, which corresponds to one 

 part of augite, four of labradorite, eight of leucite. This 



mixture is placed in the crucible at white heat, and 

 transformed into a homogeneous glass. As soon as the 

 fusion of the chemical compounds has taken place the 

 temperature is lowered, and the vitreous mass is kept 

 for forty-eight hours at the temperature of molten steel. 

 The leucite crystals are formed during this first stage 

 of the proceedings, which evidently corresponds to the 

 first phase of the consolidation of eruptive rocks. 



The mass is maintained for another forty-eight hours 

 at the temperature of molten copper, and the whole 

 residue left after the formation of the leucite crystals in 

 the former phase is transformed into microliths of augite 

 and labradorite, and octahedrons of magnetite and 

 picotite. Now let us compare, after this double anneal- 

 ing, the microscopical preparation cf the synthesis with 

 natural lava ; not only are the same minerals reproduced 

 by this process of fusion of dry materials, but the order 

 of their formation and the proportion of their con- 

 stituents are identical ; this analogy can be traced even 

 in the details of crystallographical forms. Large leucite 

 crystals are present showing all the characteristics of this 

 mineral as found in Vesuvian lava, and grouped around 

 them are the microlilhs of the second stage, augite and 

 labradorite. Finally, as in the natural rock, the leucite 

 contains enclosures of magnetite and picotite, which are 

 the most ancient minerals. 



Let us take for our second example the synthesis of 

 basalt, one of the most widely diffused types of the vol- 

 canic series, anent whose origin numerous hypotheses 

 have been advanced. It is well known that basalt is 

 composed of three essential minerals — olivine, augite, 

 and labradorite. The olivine appears in the natural 

 rock as a crystal of the first consolidation. 



As in the case of leucotephrite, a mixture of chemical 

 elementsor of pulverised mineralsis formed, corresponding 

 to the average composition of a basalt rich in olivine. 

 The mixture is composedof three partsof this mineral, two 

 of augite,'and three of labradorite. It is first transformed 

 into an almost homogeneous black glass, and maintained 

 at red white heat for forty-eight hours. On examination 

 of a thin section of this glass after this annealing at high 

 temperature, large crystals of olivine are at once detected. 

 These are embedded in a vitreous mass, in which little 

 octahedrons of massicolite and of picotite are discovered, 

 together with a few scarce crystals of augite. 



We now have to induce the formation of the micro- 

 liths of the second consolidation round the divine 

 crystals formed during the first phase. For this pur- 

 pose the crucible is kept for forty-eight hours at a cherry- 

 red heat, and after this annealing a paste is obtained 

 composed of microliths of labradorite and augite, of 

 magnetite, and of the glassy residue of this crystallisa- 

 tion. Thus in this second phase we have reproduced 

 the microlithic structure. The basalts obtained by these 

 operations can scarcely be distinguished from the natural 

 rock, and these few grammes of cleverly manipulated 

 substance supply us with - a most convincing proof of the 

 purely igneous formation of the rock. 



We could mention here whole series of remarkable 

 experiments carried out by MM. Fouque' and Levy, 

 whence we have obtained these two syntheses. All 

 the contemporary eruptive rocks have been thus recon- 

 stituted ; andesites, labradorites, basalts, limburgites, 

 nephelinites, tephrites, leucite rocks, peridodites, and 

 labradorites with ophitic structure. We will limit our- 

 selves however to proving, by one last example, how 

 these synthetic processes succeed in throwing a direct 



