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On Laboratory Arts. Richard Threlfall
Читать онлайн.Название On Laboratory Arts
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isbn 4064066106201
Автор произведения Richard Threlfall
Жанр Языкознание
Издательство Bookwire
§ 10. On the Choice of Sizes of Glass Tube. —
It will be found that for general purposes tubes about one-quarter inch in inside diameter, and from one-twentieth to one-fortieth of an inch thick, are most in demand. Some very thin soda glass of these dimensions (so-called "cylinder" tubes) will be found very handy for many purposes. For physico-chemical work a good supply of tubing, from one-half to three-quarters of an inch inside diameter, and from one-twentieth to one-eighth inch thick, is very necessary. A few tubes up to three inches diameter, and of various thicknesses, will also be required for special purposes.
Thermometer and "barometer" tubing is occasionally required, the latter, by the way, making particularly bad barometers. The thermometer tubing should be of all sizes of bore, from the finest obtainable up to that which has a bore of about one-sixteenth of an inch. Glass rods varying from about one-twentieth of an inch in diameter up to, say, half an inch will be required, also two or three sticks of white enamel glass for making joints.
To facilitate choice, there is appended a diagram of sizes from the catalogue of a reliable German firm, Messrs. Desaga of Heidelberg, and the experimenter will be able to see at a glance what sizes of glass to order. It is a good plan to stock the largest and smallest size of each material as well as the most useful working sizes.
§ 11. Testing Glass. —
"Reject glass which has lumps or knots, is obviously conical, or has long drawn-out bubbles running through the substance." If a scratch be made on the surface of a glass tube, and one end of the scratch be touched by a very fine point of fused glass, say not more than one-sixteenth inch in diameter, the tube, however large it is (within reason), ought to crack in the direction of the scratch. If a big crack forms and does not run straight, but tends to turn longitudinally, it is a sign that the glass is ill annealed, and nothing can be done with it. If such glass be hit upon in the course of blow-pipe work, it is inadvisable to waste time upon it; the best plan is to reject it at once, and save it for some experiment where it will not have to be heated.
The shortest way of selecting glass is to go to a good firm, and let it be understood that if the glass proves to be badly annealed it will be returned. Though it was stated above that the glass should not be distinctly conical, of course allowance must be made for the length of the pieces, and, on the other hand, a few highly conical tubes will be of immense service in special cases, and a small supply of such should be included.
The glass, as it is obtained, should be placed in a rack, and covered by a cloth to reduce the quantity of dust finding its way into the tubes. It has been stated by Professor Ostwald that tubes when reared up on end tend to bend permanently. I have not noticed this with lead glass well supported. Each different supply should be kept by itself and carefully described on a label pasted on to the rack, and tubes from different lots should not be used for critical welds. This remark is more important in the case of soda than of lead glass.
In the case of very fine thermometer tubes it will be advisable to cover the ends with a little melted shellac, or, in special cases, to obtain the tubes sealed from the works. Soda glass can generally be got in rather longer lengths than lead glass; the longer the lengths are the better, for the waste is less.
It is useful to be able to distinguish the different kinds of glass by the colour. This is best observed by looking towards a bright surface along the whole length of the tube and through the glass. Lead glass is yellow, soda glass is green, and hard glass purple in the samples in my laboratory, and I expect this is practically true of most samples. [Footnote: Some new lead glass I have is also almost purple in hue. If any doubt exists as to the kind of glass, it may be tested at once in the blow-pipe flame, or by a mixture of oils of different refractive indices, as will be explained later.]
§ 12. The question of the solubility of glass in reagents is one of great importance in accurate work, though it does not always meet with the attention it deserves. It is impossible here to go into the matter with sufficient detail, and the reader is therefore referred to the Abstracts of the Chemical Society, particularly for the years 1889 and 1892. The memoir by F. Kohlrausch, Wied. Ann. xliv., should be consulted in the original. The following points may be noted. A method of testing the quality of glass is given by Mylius (C. S. J. Abstracts, 1889, p. 549), and it is stated that the resistance of glass to the action of water can generally be much increased by leaving it in contact with cold water for several days, and then heating it to 300° to 400° C. This improvement seems to be due to the formation of a layer of moist silica on the surface, and its subsequent condensation into a resisting layer by the heating. Mylius (C. S. J. Abstracts, 1892, p. 411), and Weber, and Sauer (C. S. J. Abstracts, 1892, p. 410) have also shown that the best glass for general chemical purposes consists of
Silica, 7 to 8 parts
Lime, 1 part
Alkali, 1.5 to 1.1 parts.
This is practically "Bohemian" tube glass.
The exact results are given in the Berichte of the German Chemical Society, vol. xxv. An excellent account of the properties of glass will be found in Grove's edition of Miller's Elements of Chemistry.
§ 13. Cleaning Glass Tubes. —
This is one of the most important arts in chemistry. If the tubes are new, they are generally only soiled by dust, and can be cleaned fairly easily—first by pushing a bit of cotton waste through with a cane, or pulling a rag through with string—and then washing with sand and commercial hydrochloric acid. I have heard of glass becoming scratched by this process, and breaking in consequence when heated, but have never myself experienced this inconvenience. In German laboratories little bits of bibulous paper are sometimes used instead of sand; they soon break into a pulp, and this pulp has a slightly scouring action.
As soon as the visible impurities are removed and the tube when washed looks bright and clean, it may be wiped on the outside and held perpendicularly so as to allow the water film to drain down. If the tube be greasy (and perhaps in other cases) it will be observed that as the film gets thinner the water begins to break away and leave dry spots. For accurate work this grease, or whatever it is, must be removed; and after trying many plans for many years, I have come back to the method I first employed, viz. boiling out with aqua regia.
For this purpose, close one end of the tube by a cork (better than a rubber bung, because cheaper), and half fill the tube with aqua regia; then, having noted the greasy places, proceed to boil the liquid in contact with the glass at these points, and in the case of very obstinate dirt—such as lingers round a fused joint which has been made between undusted tubes—leave the whole affair for twelve hours. If the greasiness is only slight, then simply shaking with hot aqua regia will often remove it, and the aqua regia is conveniently heated in this case by the addition of a little strong sulphuric acid.
The spent aqua regia may be put into a bottle. It is generally quite good enough for the purpose of washing glass vessels with sand, as above explained.
However carefully a tube is cleaned before being subjected to blowpipe operations, it will be fouled wherever there is an opening during the process of heating, unless the extreme tip only of an oxidising flame be employed. Even this should not be trusted too implicitly unless an oxygas or hydrogen flame is employed.
When a tube or piece of apparatus has been cleaned by acid, so that on clamping it vertically, dry spaces do not appear, it may be rinsed with platinum distilled water and left to drain, the dust being, of course, kept out by placing a bit of paper round the top. For accurate work water thus prepared is to be preferred