ent. Oxide of lead enters into its composition to the amount of one third of its weight, or more, and by its presence gives that proportion of refractive and dispersive power, which makes the glass valuable in conjunction with crown or plate: this it does in consequence of its own powerful action on light; and it makes the glass-heavy also, because of its own great specific gravity. A third property belonging to it, namely, its high fluxing or dissolvent powers, it also confers upon the glass. New these three properties are unfortunately very conducive to the formation of striæ. If the least difference in composition exists between one part and another it becomes evident, because of the great difference between the qualities of the oxide of lead and the other ingredients; and a variation in proportions which in crown or plate glass would produce no sensible effect on the naked eye, would, in flint glass, form strong strife. Hence it is required that the mixture be in this case far more perfect than in the other glasses; and yet it unfortunately happens that every thing tends to make it much less so. The oxide of lead is so heavy a material, and at the same time so fusible, that it melts and sinks to the bottom, leaving the lighter materials to accumulate at the top: and so imperfect are the means of mixture, under ordinary circumstances, that glass of very different specific gravity is procured from the bottom and top of the same crucible. The following are some cases of this kind, from pots containing glass not more than six inches in depth, made from the usual materials, and retained at a full heat for twenty-four hours:—
| Top | 3.38 | 3.30 | 3.28 | 3.21 | 3.15 | 3.73 | 3.85 | 3.81 | 3.31 | 3.30 |
| Bottom | 4.04 | 3.77 | 3.85 | 3.52 | 3.80 | 4.63 | 4.74 | 4.75 | 3.99 | 3.74 |
These differences are great, and selected for illustration; but from appearances there is little reason to doubt that the same state of things, though not to such an extent, occurs in every pot of flint glass made in the ordinary way.
8. Another curious illustration of the predominance of oxide of lead at the bottom is shown in many of our specimens, which have been broken through vertically: they have been affected by sulphuretted vapours and tarnished; but the tarnish has occurred only at the bottom, where the lead is abundant, and is there very strong, whilst there is no appearance of it towards the top.
