there must be instability, where the bonding material of the internal particles of the glass is in different states of hardness; so that there is no difficulty in conceiving how a gas globe could break apparently spontaneously, for a portion of it which was not fairly toughened might be exposed to a somewhat sudden rise of temperature, produced, it may be, from a draught blowing the flame upon that particular spot. Articles such as saucers, made of glass, which, being flat, or nearly so, can be plunged into the tempering liquid with great rapidity, are usually tempered all over, and these, when toughened, can be thrown about and allowed to fall on hard floors with impunity, thus proving the facts which I have endeavoured to establish. I hope to be able to continue my investigations, and should they be worth anything, will give the results of them to the public. Before quitting this subject, I shall make a few remarks upon the process for toughening glass, which is said to have been purchased by the Prussian Government. This process is described as consisting in the application of superheated steam to the glass, brought up to a temperature near to its melting point. Having facilities for making experiments of this kind, I have had them tried with great care, but in no case have I met with a satisfactory result. This probably is owing to the fact, that I did not comply strictly with the condition of the experiments performed by the German chemist who is said to have made the invention, nor do I see from analogy how this process is likely to effect a change in the glass similar to that arising from M. de la Bastie's dipping process. If glass, instead of being taken from the annealing kiln at the proper time, be left exposed in the hot part of it, at a temperature just below that at which it softens, it will be found to become gradually opaque on its surface. Some experiments were performed many years ago by RA(C)aumur, who exposed pieces of glass, packed in plaster of Paris, to a red heat, which became gradually opaque, and lost altogether the character of glass, the texture of their material becoming crystalline, and also effected by sudden changes of temperature. Glass treated in this way was called RA(C)aumur's porcelain. All glasses do not undergo this change with equal rapidity, and some do not experience it at all; but the commoner kinds, such as bottle glass, are the best to experiment upon, for the more alumina that it contains--and it is known that bottle glass contains a considerable quantity--the more readily does it undergo this change, which is called _devitrification_. In what it consists, is not at present well understood, but it offers a field for investigation, which may produce results of very considerable benefit to manufacturers of glass.