A late and intelligent writer (see Parkes' Chemical Essays, vol. v. p. 208) remarks, that there is something very mysterious respecting the action of the diamond in cutting glass; end he informs us, that if a sheet of glass, cut with a diamond, be examined before it is actually bro ken into its intended divisions, it will be seen that it is en tirely cut through, except at the uppermost surface. If this -statement be correct, the operation of the diamond might well be pronounced mysterious ; but we can assure our readers, that we have had occasion to examine the fissures or clefts produced by a diamond, and we have always found, that they commence on the upper side of the glass where the diamond is applied, and extend gradually down wards. This may be distinctly seen, by reflecting light from the separated faces, and the progress of the fissure downwards will be marked by the change produced upon the reflected light. We conceive, therefore, that there is no mystery whatever in the action of the diamond. A piece of soft and smooth wood will cut a plate of coagu lated isinglass without any difficulty ; a piece of steel will, with the same ease, cut a plate of wood ; and, in like man ner, a diamond will cut a plate of glass with the same fa cility, because it is as much harder than the glass, as the wood is harder than the isinglass, or as the steel is harder than the wood.
In order that the wood may cut the isinglass, or the steel the wood, the cutting point must be smooth, otherwise the surfaces to be cut will be torn up or scratched, instead of being really divided. For the same reason, the diamond must have a smooth natural point, or solid angle ; for if a piece broken from a diamond is employed, it will only scratch the glass.
Mr Atwood, in a communication to Mr Parkes, informs us, that a good cutting diamond should be of a regular rhom boidal form, or have one regular smooth edge and rhomboi dal point. The least deviation of the diamond from a par ticular position and inclination will prevent the cut from taking place. The workman, who is guided altogether by his ear, judges by the peculiar creaking of the glass ; and if he does not hear this particular noise, he varies the po sition of the diamond till it occurs, and then draws it on wards. The diamond apparently wears down at the cut ting point by long use, though it will last an ordinary gla zier for many years.
" When the cut is perfectly good," says Mr Atwood, " it should be an internal fracture, unaccompanied with any scratch, or any visible impression on the surface whatever; for in proportion as any such superficial injury is produced, the completeness of the internal fracture is diminished. This fracture, therefore, which is called a cut, froth its re semblance thereto in its effects, as also in the similarity of its appearance to a real cut, produced on any other sub stance by a sharp edged instrument, but which has closed again, (the expression being further countenanced, by the sharp form of that part of the diamond which comes in
.contact with the glass,) is really no cut at all, nor does the diamond so much as enter the surface." If this reasoning be just, it will follow, that a good diamond is not capable of making any impression upon the surface of a plate of glass over which it is drawn, and that it acts where it is not, and does not act where it is. It would require singu lar ingenuity to support such paradoxes.
M i r Shaw, an ingenious watchmaker in Leicestershire, being desirous of giving some assistance to a relation of his own, who was a glazier, and who, by a paralytic affec tion, was unable to pursue his trade, invented a method of fitting up a diamond, by widch any person can cut glass as perfectly as the most experienced glazier. This invention was made about a vcar ago, and its advantages have been secured by a patent. " By the use of one of these instru ments," says Mr Atwood, " a person not at all accustomed to a diamond, may produce a perfect cut over a table of glass so uneven in its surface, that the most skilful work man, with a common glazier's diamond, would not be able to produce a cut of any kind upon it, This consists in giving the diamond perfect play, and at the same time af fording it such guide and support, as effectually prevent it from being affected by the unsteadiness of the hand, or unevenness of surface in respect of its inclination to the plane of the table; whereby the diamond, being well set or mounted in its carriage, becomes equally certain in the hands of every person." Parkes' Chemical Essays, vol. v. This patent diamond is represented in Plate CCLXXV. Sce the description of this Plate at the end of the volume.
When a plate of glass is very thick, it cannot be easily cut by the diamond; but the same effect may be produced by the proper application of a hot iron. The pact of the glass where the cut is to commence must be marked by a file, and a hot iron must be applied to the place, and held a little below the groove which the file has made. In a few minutes the glass will give a crack, and the iron must be instantly removed. The iron must be again applied a little below the termination of this crack, sometimes one tenth or two-tenths of an inch distant from it, and in the line in which the cut is to be made, and the crack will ad vance in the direction of the iron. By again applying the iron in a similar manner, the crack may be conducted in any required direction. If the glass is to be cut in the form of a curve, then the hot iron must always be held very near the termination of the crack, in order that it may advance by short steps. The fissure is often most complete on the side of the glass where the iron is ap plied, and it is sometimes advisable to apply the iron to the opposite side. By this means, we have often cut plates of glass four-tenths of an inch thick with the utmost accuracy.