M. Wyrouboff mi the Colouring -matter of Fluorides. 385 



on a small scale, but subsequently at the manufactory of St. 

 Gobain. 



The various specimens prepared were obtained by taking ordi- 

 nary flint-glass as the type, and successively replacing minium 

 or carbonate of potash by carbonate of thallium in equivalent 

 quantities. One specimen was made from sand, 300, pure car- 

 bonate of thallium, 400, and carbonate of potash, 100. 



The glass resulting from this mixture readily fuses and refines, 

 but the cooled mass is not homogeneous. The layers which oc- 

 cupy the upper part of the crucible have a less-pronounced yel- 

 lowish colour, and a lower density, than the inferior layers, which 

 are moreover richer in thallium. 



Another specimen was obtained with the proportions : — sand, 

 300; minium, 200; pure carbonate of thallium, 335. 



This mixture melts and is refined more easily than the prece- 

 ding ; the mass is perfectly homogeneous, but presents a yellow- 

 ish tint, which, however, is agreeable and very brilliant. The 

 density 4'235, and refractive index 1*71 (for the yellow ray), are 

 greater than the corresponding elements of all known glasses. 

 By varying the proportions in the mixture, Lamy obtained glasses 

 whose densities varied from 4*235 to 5*625, and refractive in- 

 dices from 1*71 to 1*965. 



Lamy concludes, from the results of his experiments, (1) that 

 thallium replaces potassium better than lead in the composition 

 of glass, which is a new proof of the rank assigned to the new 

 metal in the classification ; (2) that thallium gives to glass a yel- 

 lowish, as sodium does a greenish colour ; (3) that thallium-glass 

 is denser and more refriugent than potassium-glass; (4) that 

 these properties of thallium-glass may doubtless be advanta- 

 geously used in making certain optical glasses or artificial pre- 

 cious stones. 



There are in nature a number of minerals whose colour is some- 

 times very intense, and which become completely decolorized when 

 they are heated to a certain temperature. The cause of this phe- 

 nomenon is unknown; it is especially marked in the case of the 

 fluorides, which Wyrouboff* has investigated; from the results 

 of his investigations he concludes 



] . That the fluorides are of aqueous origin. 



2. That the colouring-substances are various carburets of hydro- 

 gen, probably arising from bituminous limes, which by their de- 

 composition have furnished the materials which have concurred 

 in the formation of the fluoride. 



3. That the odour of the fluorine of Wulsendorff is due to 

 the presence of a carburetted hydrogen accumulated in the fis- 

 sures of the rock. 



* Bulletin de la Societe Chimique, May 1866. 

 t Phil. Mag. S. 4. vol. xx. p. 515. 

 Phil Mag. S. 4. Vol. 32. No. 217. Nov. 1866. 2 C 



