Page:Popular Science Monthly Volume 8.djvu/332

318 Upon the supposition that glass contracts in cooling, he bases the construction and working of his moulds, in which glass-ware is pressed, and the success of their operation assures him that he is working upon a safe conclusion.

For further assurance, he replaces an article of glass-ware, when cold, in the mould in which it was originally pressed, and finds that it easily returns to its place, and fails to fill the mould. With his calipers he measures carefully the glass and the mould, and finds the shrinkage has been about one-fiftieth of the original bulk.

He remembers that he has on his book-shelf a work by Apsley Pellatt, in which that careful and accurate observer states as follows: "A piece of unannealed barometer-tube, forty inches long, measured when just drawn, will become about one-fourth of an inch shorter if annealed; whereas, if quickly cooled without annealing, it will only contract about one-eighth of an inch." It must be borne in mind that the barometer-tube, when just drawn, at the time when it is first measured, has already considerably cooled from a fluid state of the glass, and has effected a part of its shrinkage, although not yet solid or rigid in its structure.

As the gray cast-iron before mentioned is said to expand at the moment of solidifying, but afterward to contract with farther cooling, he experiments with the view to ascertain if an analogous action takes place in glass. He tests the cooling of a crucible full of this molten material, to note if at any time in the cooling process an expansion of its substance takes place. Even from the first moment, when the crucible is taken from the extreme heat of the furnace, he finds that the surface of the vitreous mass takes a concave form, this concavity becoming more considerable as the cooling process goes on.

If there were expansion at the moment of solidifying, the mass would then bulge upward, that is, the concave line of the surface would be disturbed. But, as the concavity of this surface constantly and uninterruptedly increases until the mass becomes cold, he finds renewed proof of the shrinkage of solidifying glass.

His ordinary observation thus confirmed by careful tests and by other authority, he feels that there is no possibility for him to be in error in regard to this contraction of glass, which he sees constantly going on.

When he reads, in the article of that the exterior coating produced by the immediate chill of the surface of the glass "prevents the interior atoms from expanding and arranging themselves in such a way as to give the glass a fibrous nature, as they would if the glass were allowed to cool very gradually," he tries to remember an instance, where, in some very perfectly-annealed glass, there has been an indication of such fibrous nature, but finds himself unable, in his own experience, or in that of his