250 Some Account of the late M. Guinand. 



lead, he supposes that, from some cause or other, certain particles of 

 the lead which composes his vitreous matter separate from it, and 

 appear on its surface in their metallic state ; that becoming again 

 oxydated by contact with the air, or re-calcined after being re- 

 susciiafed, (to use his own expression,) they combine with the 

 vitreous matter on which they rest, and thus form in the place 

 where they occur, that glass of greater density which appears on 

 the surface in the form of drops. The specific gravity of this 

 substance draws it to the bottom of the crucible ; but in descend- 

 ing more or less slowly, according to the temperature of the fur- 

 nace, it leaves in its passage a train which occasions those threads 

 of glass that possess a stronger refraction. Having reached th6 

 bottom, this vitreous matter, in some degree saturated with mi- 

 nium, being a powerful solvent, attacks the substance of the 

 crucible, and forms with it a vitreous compound, of an inferior 

 density to the mass, and ascending in consequence of its specific 

 levity, produces those cylinders or tubes formed of a less refrac- 

 tive glass. Lastly, when this solvent, by melting the substance 

 of the crucible, especially that of the bottom, has detached from 

 it a grain of saiid or baked clay, this half molten grain rises and 

 floats in the mass in an oblique direction, because, being still 

 attached to a part of the vitreous matter which it has produced, 

 it is not actuated on all its points to ascend with equal rapidity *. 



Whatever may be thought of this explanation, the original 

 cause of the non- homogeneity of strongly refractive glass being 

 once ascertained, the question was, how to remedy it ; and it was 



* This explanation having been communicated to M. Breguet^ a Swiss artist, 

 who was subsequently attached to the Board of Longitude at Paris, but who 

 at this period was still residing in his own country, he was of opinion, that to 

 obtain homogeneous flint-glass, it would be sufficient to melt it in a covered 

 crucible of platina; he consequently transmitted to M. Guinand a small plate 

 of that precious metal, but the latter did not coincide in M. Breguet's opinion . 

 he foresaw that the solvents would act sufficiently on the metal to colour the 

 glass, and his conjecture was justified by an experiment made in a very small 

 crucible. However, when M. Breguet went to reside at Paris, he announced 

 to thB public, tliat the cause of the defects in flint-glass being known, as well 

 as the remedy, it wbuld thenceforward be easily obtained in a perfect state. 



