3i8 THE POPULAR SCIENCE MONTHLY. 



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 cali- 

 pers he measures carefully the glass and the mould, and finds the 

 shrinkage has been about one-fiftieth of the original bulk. 



He remembers tliat he has on his book-shelf a work ' by Apsley 

 Pellatt, in wliich 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 wull 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 uf a crucible full of this 

 molten material, to note if at any time in the cooling process an ex- 

 pansion 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 



goino: on. 



When he reads, in the article of The Popular Science Month- 

 ly, 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 gradu- 

 ally," 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 

 ' " Curiosities of Glass-making," by Apsley Pellatt, London, 1849, p. 63. 



