TO MAKE SMALL GLASS MICROSCOPIC GLOBULES.
Take two rods of glass, one in each hand, place their extremities close to each other, and in the purest part of the flame; when you perceive the ends to be fused, separate them from each other; the heated glass following each rod, will be finer, in proportion to the length it is drawn to, and the smallness of the rod; in this manner you may procure threads of glass of any degree of fineness. Direct the flame to the middle of the thread, and it will be instantly divided into two parts. When one of the threads is perfectly cool, place it at the extremity of the flame, by which it will be rendered round; and, if the thread of glass be very fine, an exceeding small globule will be formed. This thread may now be broke off from the rod, and a new one may be again drawn out as before, by the assistance of the other glass rod.
The small ball is now to be separated from the thread of glass; this is easily effected by the sharp edge of a piece of flint. The ball should be placed in a groove of paper, and another piece of paper be held over it, to prevent the ball from flying about and being lost. A quantity of globules ought to be prepared in this manner; they are then to be cleaned, and afterwards placed in the cavities of the tripoli, by means of a delicate pair of nippers. The globules are now to be melted a second time, in order to render them completely spherical; for this purpose, bring one of the cavities near the extremity of the flame, directing this towards the tripoli, which must be first heated; the cavity is then to be lowered, so that the flame may touch the glass, which, when it is red hot, will assume a perfect globular form; it must then be removed from the flame, and laid by; when cold, it should be cleaned, by rubbing between two pieces of white paper. Let it now be set in a brass cap, to try whether the figure be perfect. If the object be not well defined, the globule must be thrown away. Though, if it be large, it may be exposed two or three times to the flame. When a large globule is forming, it should be gently agitated by shaking the tripoli, which will prevent its becoming flat on one side. By attending to these directions, the greater part of the globules will be round and fit for use. In damp weather, notwithstanding every precaution, it will often happen, that out of forty globules, four or five only will be fit for use.