In order to carry out stirring operations in the presence of a gas or mixture of gases other than air, some form of gland or seal may be necessary where the stirrer passes through the bearing in which it runs. A flask to which is fitted a stirrer and gas seal is shown in section by _b_, Fig. 8. The liquid used in this seal may be mercury, petroleum, or any other that the experimental conditions indicate. [Illustration: Fig. 8] If the bearing for a stirrer is made of glass tube, it is desirable to lubricate rather freely; otherwise heat will be produced by the friction of the stirrer and the tube will probably crack. Such lubrication may be supplied by turning out the top of the bearing tube and filling the turned-out portion with petroleum jelly mixed with a small quantity of finely ground or, better, colloidal graphite, and the bearing should also be lubricated with the same composition. Care should be taken not to employ so soft a lubricant or so large an excess as to cause it to run down the stirrer into the liquid which is being stirred. _Leading a Crack._--It sometimes happens that a large bulb or specially thin-walled tube has to be divided. In such a case it is scarcely practicable to use the method recommended for small tubes on page 12, but it is quite easy to lead a crack in any desired direction. A convenient starting point is a file cut; this is touched with hot glass until a crack is initiated. A small flame or a bead of hot glass is now used to heat the article at a point about a quarter of an inch from the end of the crack and in whatever direction it has to be led. The crack will now extend towards the source of heat, which should be moved farther away as the crack advances. In this manner a crack may be caused to take any desired path and can be led round a large bulb. _Cutting Glass with the Diamond._--Slips of window-glass can be used in place of glass rod for some purposes, and as cutting them involves the use of the glaziers' diamond or a wheel-cutter, they may well be mentioned under this heading. In cutting a sheet of glass with the diamond, one needs a flat surface on which to rest the glass, and a rule against which to guide the diamond. The diamond should be held in an almost vertical position, and drawn over the surface of the glass with slight pressure. While this is being done the angle of the diamond should be changed by bringing the top of the handle forward until the sound changes from one of scratching to a clear singing note. When this happens the diamond is cutting. A few trials will teach the student the correct angle for the diamond with which he works, and the glass, if properly cut, will break easily. If the cut fails it is better to turn the glass over and make a corresponding cut on the other side rather than make any attempt to improve the original cut. The diamond is seldom used for cutting small glass tubes.