538 SCIENTIFIC RECORD FOR 188-2. 



between the black rings of the axial interference figures seen in mono- 

 chromatic converging polarized light ; he has applied this method to 

 muscovite and more recently to the uniaxial mineral brucite. 



3. CHEMICAL MINERALOGY. 



In the direction of chemical mineralogy the most important advances 

 that have been made are in the artificial formation of minerals. This 

 is a subject to which the French mineralogists and chemists have es- 

 pecially devoted themselves, and the results they have attained are 

 most remarkable. Some of the more recent achievements in this line 

 are the making of analcite, leucite, gaylussite, opal, wurtzite, scorodite, 

 &c. It would not be appropriate to discuss here the methods employed 

 in these or similar cases, but attention may properly be called to a work 

 on the subject by Fouqu6 and Levy entitled Synthese des Mineraux et 

 des Roches. These authors have already themselves performed some of 

 the most important of the synthetic experiments, and they have now 

 published a volume of 420 pages octavo, in which all previous results 

 are brought together. The work is admirably constructed, giving a 

 summary of the methods employed, a discussion of the principles in- 

 volved, as well as of the value of the synthetic method, and further, a 

 statement of the artificial production of the individual species, with full 

 references to previous experimenters and observers. This valuable work 

 cannot but act as a stimulus to further the develoj)ment of a most im- 

 portant branch of mineralogy, which has a wide bearing not only upon 

 this science itself, but beyond, upon geology and chemistry. 



The analytical method has also, as of old, been followed most indus- 

 triously in the past two years, and the result has been to settle the 

 composition of some minerals in cases where it has long been doubtful, 

 and to throw light on many obscure points. Among the many cases 

 coming under this head may be mentioned the work of Penfield on 

 amblygonite, in which he shows that the many varieties of the species 

 all conform to the same general formula, although differing in the amount 

 of fluorine and water they contain, the latter increasing as the former 

 diminishes. He explains this variation by assuming that hydroxyl 

 comes in to replace the fluorine to a greater or less extent. The same 

 chemist has also settled the doubtful composition of monazite. Early 

 analj'ses showed the presence of a considerable amount of thorium, while 

 some later ones have aflbrded no thorium at all. Penfield has proved 

 that the mineral is a simple phosphate of the metals of the cerium 

 group, the thorium being due to admixed thorite. He analyzed sam- 

 ples of monazite from different American localities, and showed that 

 while all afforded thorium in widely different amounts, the silica pres- 

 ent also in each case was sufficient to unite with the thorium ; more- 

 over, thin sections showed the existence of a foreign mineral which 

 gelatinized with acids. The doubtful relations of the minerals of the 

 cryolite group have been made clear by the chemical work of Brandl 



