Jan. 17, 1889] 



NA TCJRB 



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was being thus opened up, analysis and the methods 

 of investigation had not attained to the perfection which 

 they enjoy at the present day ; on the other hand, the 

 prejudices which held sway in the infancy of geology 

 increased the obstacles, and these were not surmounted 

 until half a century later. We need not be detained here 

 by the brilliant period of mineral synthesis which fol- 

 lowed close on the development of chemistry and mine- 

 ralogy. It is sufficient to cite the names of Ebelmen, 

 Rosej Mitscherlich, and Senarmont, to recall those re- 

 markable results in the artifical reproduction of minerals. 

 Hut the researches of these savants related chiefly 

 to the synthesis of isolated species, and not to rocks 

 which are aggregates of mineral species. Speaking 

 generally, it may be said that their researches were 

 essentially mineralogical, and bore but subordinately on 

 iithology. Nevertheless, the researches of these skilful 

 experimentalists shed much light upon geological prob- 

 lems. They also show us that, as the mineral sciences 

 progress, we are led to seek, by experimental methods, 

 th? most complete interpretation of the phenomena ot 

 Nature. Finally, in 1866, Daubrde led the way to the 

 reproduction of crystalline rocks by simple fusion. This 

 is the method which has been since taken up and deve- 

 loped by MM. Fouqud and Michel L^vy. The researches 

 of Daubr(5e, to which we refer, are those in which he 

 sought to reproduce by fusion certain meteoric stones 

 characterized by the absence of a fel spathic element. By 

 fusing Iherzolite', a terrestrial rock which approximates in 

 composition to certain meteorites, he succeeded in ob- 

 taining products which, in the details of their structure 

 and composition, resembled the cosmical types which he 

 desired to imitate. 



While this eminent geologist thus foreshadowed the 

 researches which some years afterwards shed so brilliant 

 a lustre upon the geological laboratory of the College de 

 France, tlie synthetic methods were still encumbered by 

 hypotheses, 'it is true we had no longer to struggle 

 against the assumed influence of mysterious forces ; but 

 it was held that the reproduction of geological phenomena 

 in the laboratory would be possible only if we had an in- 

 fmite duration of time at our disposal, and dealt with 

 temperatures and masses far beyond those which we could 

 hope to command in the laboratory. It was still supposed 

 that the mineral associations in Nature were governed by 

 other laws than those which determined the combinations 

 produced by the chemist. Such prejudices would certainly 

 not have hindered Daubree from proceeding in the path 

 in which he so bravely took the first step by his synthesis 

 of meteorites ; for he, indeed, is one of those whose works 

 have largely contributed to banish such prejudices from 

 the realms of geology ; but the methods of analysis 

 which then existed did not allow us to probe the 

 _, nature of the rocks to their very base, and to compare 

 ■jk-. their intimate structure with that of the products of 

 BjB synthesis. Our laboratories were not then in possession 

 of the apparatus by means of which we can command 

 those very high temperatures, maintained during a 

 prolonged period, which such experiments require. 



The great improvements in the construction of appa- 

 ratus, and the application of the microscope to Iithology, 

 have at length enabled us to successfully attempt the 

 reproduction of all the modern volcanic rocks. Two 

 French savants, MM. Fouquc and Michel L^vy, who 

 introduced into their country the study of micrographic 

 Iithology, began in 1877 a series of synthetic experiments 

 destined to be memorable in the annals of science. One 

 of them had already acquired a just reputation by his 

 remarkable researches on volcanic phenomena, carried 

 on in various classical regions : he was familar with all 

 the secrets of the chemical analysis of minerals— a de- 

 partment which he had enriched by the most ingenious 

 and useful methods. The other, prepared by the severe 

 studies of the high French schools, had undertaken, with 



brilliant success, the examination of minerals by their 

 optical properties : he had carried exact methods into 

 micrography, far beyond what others had done, and he 

 was known by his researches on the eruptive rocks of the 

 older series. 



By their joint labours, MM. Fouqud and Ldvy have to 

 some extent systematized and co-ordinated the facts re- 

 lative to the chronological succession of the crystals in 

 eruptive rocks, and have discovered many of the details 

 which we have already noticed in explaining the results of 

 the analyses of lavas. It is to this happy association of 

 talent, to this fruitful collaboration, that we owe those 

 beautiful discoveries which have given such celebrity to 

 the laboratory of the College de France, and to which 

 it is an honour for me to render homage before an audience 

 ever ready to welcome scientific progress, and in a place 

 where the immortal Faraday once brought forward, with 

 generous enthusiasm, the admirable researches of Ebelmen 

 in connection with mineral synthesis. 



We have already indicated the data upon which these 

 savants relied in their researches : they are furnished by 

 chemical and mineralogical analysis. One point, how- 

 ever, not yet touched upon, lies at the base of their 

 general procedure. Theory would predict that the most 

 ancient crystals in an igneous rock should be thos3 

 which are the least fusible. And this, speaking in general 

 terrns, is really what we observe : the minerals of the first 

 period of crystallization are those which occupy the 

 lowest degrees in the scale of fusibility. The constituent 

 mineral species of lavas have appeared at successive 

 periods, as the temperature diminished, according to 

 their relative degrees of fusibility. These facts, proved 

 in detail by microscopic analysis, served as the point of 

 departure in the experiments of MM. Fouque and L^vy. 

 Their process rests, morever, upon a fact which James 

 Hall foresaw : namely, that the fusion of a rock pro- 

 duces a glass which is more easily fusible than any of 

 the constituent crystalline species of the rock. Now, if 

 we fuse a natural aggregate of minerals and subject the 

 glass produced by this fusion to a series of diminishing 

 temperatures, but always higher than that of the fusing- 

 point of the vitreous mass, the minerals, which can 

 crystallize from this magma should make their appear- 

 ance one after another, and the less fusible should be the 

 first to separate. These crystals will be united and 

 moulded round by those of which the fusibility is 

 higher, and which will appear in turn as the temperature 

 decreases. Without dwelling on the technical details of 

 the api)aratus, it suffices to say that, by aid of the furnaces 

 and bellows, which MM. Fouqud and Ldvy employ in 

 their syntheses, we can obtain all degrees of temperature, 

 from a dull red to a dazzling white heat, and can maintain 

 a given temperature constant for an unlimited period. 



Into the furnace we introduce a platinum crucible of a 

 capacity of about 20 cubic centimetres, containing the 

 mixture of mineral substances which by fusion and recutt 

 are to be transformed into the rock. First, by aid of 

 special arrangements, we subject it for a long time to a 

 glowing white heat, and the mixture is converted into a 

 glass. By regulating the admission of gas and air, and by 

 uncovering the furnace, the temperature is lowered to an 

 orange heat— the fusing-point of steel. By raising the 

 crucible in the furnace, the temperature falls to a cherry- 

 red heat— the melting-point of copper. Finally, if the 

 crucible be completely removed from the furnace, it can 

 still be maintained at a temperature at which copper would 

 fuse with difficulty. 



We have thus indicited the broad lines of the operation. 

 These are the successive rccuits at diminishing tempera- 

 tures which cause the crystals to be formed in sequence, 

 commencing with the least fusible, and which enable us 

 to impart to the fused matter the texture and the mineral 

 composition of volcanic products. 



We proceed to illustrate by examples the method of 



