Some Account of the late M. Guinand. 351 * 



examine what had taken place during fusion. On the upper surface of 

 the vitreous matter there were many little semi-globules, which had the 

 appearance of drops of water, terminating by a thread or little tube of 

 greater or less depth, at the extremity of which there was a small spheri- 

 cal bulb- The cause of this appearance was, that these drops and tubes 

 consisted of a denser kind of glass than the rest. In another part, there 

 arose from the bottom of the crucible other cylinders or tubes, terminat- 

 ing also in a kind of swelling or bulb ; these had a hollow appearance, be- 

 cause they were formed of a substance less dense than the rest of the 

 glass ; and lastly, here and there were seen specks or grains ending with 

 a tail, of a substance less dense than the rest of the mass ; these, on ac- 

 count of their appearance, he denominated comets. 



Having often seen on the surface of his glass small globules of lead, he 

 supposed that certain particles of thelead which enters into the composition of 

 his vitreous matter separate from it, and appear on its surface in their metal- 

 lic state; that becoming again oxydated by contact with the air, or re-calcined 

 after being revived, they combine with the vitreous matter on which they 

 rest, and thus form that glass of greater density which appears on the surface 

 in the form of drops. The specific gravity of this substance causes it to 

 sink to the bottom of the crucible; but, in descending more or less slowly, 

 according to the temperature of the furnace, it leaves in its passage a train 

 which occasions those threads of glass that possess a stronger refraction. 

 Having reached the bottom, this vitreous matter, in some degree saturated 

 with minium, 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, form- 

 ed of a less refractive glass. Lastly, when this solvent, by melting the 

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

 it a grain of sand 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 question was, how to 

 remedy the non-homogeneity of strongly refractive glass ; and it was here, 

 in particular, that M. Guinand had great obstacles to surmount. Having, af- 

 ter many expensive trials, been so fortunate as to obtain glass of which 

 some parts were perfectly homogeneous; and, therefore, destitute of those 

 striae of which flint-glass is so rarely free, he reflected on the different cir- 

 cumstances which, in this experiment, might have contributed to the re- 

 sult, so that, in subsequent attempts, he obtained blocks of glass possess- 

 ing larger portions of homogeneous substance, and he has almost arrived at 

 a certainty of obtaining in the fusion of from two to four hundred weight 

 of glass, at least one-half of it perfectly homogeneous, and, consequently, fit 

 for optical purposes. 



M. Guinand admitted that his processes had not yet attained all the per- 

 fection which might perhaps be desired ; but as he has by these means 

 succeeded in making disks, perfectly homogeneous, of twelve, and in one 



