_Lead-Glass._--This, or "flint" glass as it is often called from the fact that silica in the form of crushed and calcined flint was often used in making the English lead-glasses, contains a considerable proportion of lead silicate. Such a glass has, usually, a particularly bright appearance, a high refractive index, and is specially suitable for the production of the heavy "cut-glass" ware. Lead-glass tubing is easy to work in the blowpipe flame, melts easily, but does not become fluid quite so suddenly as most soda-glasses; articles made from it are remarkably stable and free from tendency to spontaneous cracking, although, as is essential for all the heavy or "glass-house" work, the massive articles need annealing in the oven. The two chief disadvantages of lead-glass for laboratory work are that it is blackened by the reducing gases if held too near to the blue cone of the blowpipe flame, and that it is rather easily attacked by chemical reagents; thus ammonium sulphide will cause blackening. The effect of the reducing flame on lead is not altogether a disadvantage, however; because a little care in adjusting the blowpipe and a little care in holding the glass in the right position will enable the student to work lead-glass without producing the faintest trace of blackening. This, in addition to being a valuable exercise in manipulation, will teach him to keep his blowpipe in good order, and prove a useful aid in his early efforts to judge as to the condition of the flame. It prevents discouragement if the student does his preliminary work with the soda-glass, but he should certainly make experiments with lead-glass as soon as he has acquired reasonable dexterity with soda-glass. _Annealing._--Annealing is a process by which any condition of strain which has been set up in a glass article, either by rapid cooling of one part while another part still remains hot, or by the application of mechanical stress after cooling is relieved. Annealing is carried out by subjecting the article to a temperature just below the softening point of the glass, maintaining that temperature until the whole article has become heated through the thicker part, and then reducing the temperature very gradually; thus avoiding any marked cooling of the thinner and outer parts first. For thin glass apparatus of the lamp-blown or blowpipe-made variety in which there are no marked difference of thickness, such as joins on tubes, ordinary seals, bulbs, etc., there is little need for annealing; and even those having rather marked changes of thickness, such as filter pumps, can be annealed sufficiently by taking care that the last step in making is heating to just below visible redness in the blowpipe flame and then rotating in a sooty gas flame until covered with a deposit of carbon. The article should then be allowed to cool in a place free from draughts and where the hot glass will not come in contact with anything.