These remarks apply, with suitable modification, to all kinds of finished apparatus having two openings. For flasks and so on, it is convenient to employ a blowing apparatus, dust being avoided by inserting a permanent plug of cotton wool in one of the leading tubes. The efficiency of this method is greatly increased by using about one foot of thin copper tube, bent into a helix, and heated by means of a Bunsen burner; the hot air (previously filtered) is passed directly into the flask, bottle, or whatever the apparatus may be. This has proved so convenient that a copper coil is now permanently fastened to the wall in one of the rooms of my laboratory.
The above instructions indicate greater refinement than is in general necessary or proper for tubes that have to be afterwards worked by the blow-pipe. In the majority of cases all that is necessary is to remove the dust, and this is preferably done by a wad of cotton waste (which does not leave shreds like cotton wool), followed by a bit of bibulous filter paper. I would especially warn a beginner against neglecting this precaution, for in the process of blowing, the dust undergoes some change at the heated parts of the apparatus, and forms a particularly obstinate kind of dirt.
In special cases the methods I have advocated for removing dirt and drying without covering the damp surfaces with dust are inadequate, but an experimenter who has got to that stage will have nothing to learn from such a work as this.
I suppose a small book might easily be written on this subject but what I have to say — in accordance with the limitation imposed — will be brief. For working lead glass I never use anything but an oxygas blow-pipe, except for very large work, and should never dream of using anything else. Of course, to a student who requires practice in order to attain dexterity this plan would be a good deal too dear. My advice to such a one is — procure good soda glass, and work it by means of a modification of a gas blow-pipe, to be described directly. The Fletcher's blow-pipes on long stems are generally very inconvenient. The flame should not be more than 5 or 6 inches from the working table at most, especially for a beginner, who needs to rest his arms on the edge of the table to secure steadiness.
The kind of oxygas blow-pipe I find most convenient is indicated in the sketch. (Fig. 2) I like to have two nozzles, which will slip on and off, one with a jet of about 0.035 inch in diameter, the other of about double this dimension. The oxygen is led into the main tube of the blow-pipe by another tube of much smaller diameter, concentric with the main tube (Fig. 3, at A). The oxygen is mixed with the gas during its escape from the inner tube, which is pierced by a number of fine holes for the purpose, the extreme end being closed up. The inner tube may run up to within half an inch of the point where the cap carrying the nozzle joins the larger tube.
Fig. 2.
Fig. 3.