(7) The breaking of carbide must be done by men provided with respirators and goggles, and care taken to avoid the formation of dust.
(8) Local or other authorities will issue from time to time special regulations in regard to carbide trade premises.
The ITALIAN GOVERNMENT rules relating to the storage and transport of carbide follow in the main those of the Austrian Government, but for quantities between 300 and 2000 kilos sanction is required from the local authorities, and for larger quantities from superior authorities. The storage of quantities ranging from 300 to 2000 kilos is forbidden in dwelling-houses and above the latter quantity the storage-place must be isolated and specially selected. No special permit is required for the storage of quantities not exceeding 300 kilos. Workmen exposed to carbide dust arising from the breaking of carbide or otherwise must have their eyes and respiratory organs suitably protected.
THE PURCHASE OF CARBIDE.--Since calcium carbide is only useful as a means of preparing acetylene, it should be bought under a guarantee (1) that it contains less impurities than suffice to render the crude gas dangerous in respect of spontaneous inflammability, or objectionable in a manner to be explained later on, when consumed; and (2) that it is capable of evolving a fixed minimum quantity of acetylene when decomposed by water. Such determination, however, cannot be carried out by the ordinary consumer for himself. A generator which is perfectly satisfactory in general behaviour, and which evolves a sufficient proportion of the possible total make of gas to be economical, does not of necessity decompose the carbide quantitatively; nor is it constructed in a fashion to render an exact measurement of the gas liberated at standard temperature and pressure easy to obtain. For obvious reasons the careful consumer of acetylene will keep a record of the carbide decomposed and of the acetylene generated--the latter perhaps only in terms of burner- hours, or the like; but in the event of serious dispute as to the gas- making capacity of his raw material, he must have a proper analysis made by a qualified chemist.
Calcium carbide is crushed by the makers into several different sizes, in each of which all the lumps exceed a certain size and are smaller than another size. It is necessary to find out by experiment, or from the maker, what particular size suits the generator best, for different types of apparatus require different sizes of carbide. Carbide cannot well be crushed by the consumer of acetylene. It is a difficult operation, and fraught with the production of dust which is harmful to the eyes and throat, and if done in open vessels the carbide deteriorates in gas- making power by its exposure to the moisture of the atmosphere. True dust in carbide is objectionable, and practically useless for the generation of acetylene in any form of apparatus, but carbide exceeding 1 inch in mesh is usually sold to satisfy the suggestions of the British Acetylene Association, which prescribes 5 per cent, of dust as the maximum. Some grades of carbide are softer than others, and therefore tend to yield more dust if exposed to a long journey with frequent unloadings.
There are certain varieties of ordinary carbide known as "treated carbide," the value of which is more particularly discussed in Chapter III. The treatment is of two kinds, or of a combination of both. In one process the lumps are coated with a strong solution of glucose, with the object of assisting in the removal of spent lime from their surface when the carbide is immersed in water. Lime is comparatively much more soluble in solutions of sugar (to which class of substances glucose belongs) than in plain water; so that carbide treated with glucose is not so likely to be covered with a closely adherent skin of spent lime when decomposed by the addition of water to it. In the other process, the carbide is coated with or immersed in some oil or grease to protect it from premature decomposition. The latter idea, at least, fulfils its promises, and does keep the carbide to a large extent unchanged if the lumps are exposed to damp air, while solving certain troubles otherwise met with in some generators (cf. Chapter III.); but both operations involve additional expense, and since ordinary carbide can be used satisfactorily in a good fixed generator, and can be preserved without serious deterioration by the exercise of reasonable care, treated carbide is only to be recommended for employment in holderless generators, of which table-lamps are the most conspicuous forms. A third variant of plain carbide is occasionally heard of, which is termed "scented" carbide. It is difficult to regard this material seriously. In all probability calcium carbide is odourless, but as it begins to evolve traces of gas immediately atmospheric moisture reaches it, a lump of carbide has always the unpleasant smell of crude acetylene. As the material is not to be stored in occupied rooms, and as all odour is lost to the senses directly the carbide is put into the generator, scented carbide may be said to be devoid of all utility.
THE REACTION BETWEEN CARBIDE AND WATER.--The reaction which occurs when calcium carbide and water are brought into contact belongs to the class that chemists usually term double decompositions. Calcium carbide is a chemical compound of the metal calcium with carbon, containing one chemical "part," or atomic weight, of the former united to two chemical parts, or atomic weights, of the latter; its composition expressed in symbols being CaC_2. Similarly, water is a compound of two chemical parts of hydrogen with one of oxygen, its formula being H_2O. When those two substances are mixed together the hydrogen of the water leaves its original partner, oxygen, and the carbon of the calcium carbide leaves the calcium, uniting together to form that particular compound of hydrogen and carbon, or hydrocarbon, which is known as acetylene, whose formula is C_2H_2; while the residual calcium and oxygen join together to produce calcium oxide or lime, CaO. Put into the usual form of an equation, the reaction proceeds thus--
(1) CaC_2 + H_2O = C_2H_2 + CaO.
This equation not only means that calcium carbide and water combine to yield acetylene and lime, it also means that one chemical part of carbide reacts with one chemical part of water to produce one chemical part of acetylene and one of lime. But these four chemical parts, or molecules, which are all equal chemically, are not equal in weight; although, according to a common law of chemistry, they each bear a fixed proportion to one another. Reference to the table of "Atomic Weights" contained in any text-book of chemistry will show that while the symbol Ca is used, for convenience, as a contraction or sign for the element calcium simply, it bears a more important quantitative significance, for to it will be found assigned the number 40. Against carbon will be seen the number 12; against oxygen, 16; and against hydrogen, 1. These numbers indicate that if the smallest weight of hydrogen ever found in a chemical compound is called 1 as a unit of comparison, the smallest weights of calcium, carbon, and oxygen, similarly taking part in chemical reactions are 40, 12, and 16 respectively. Thus the symbol CaC_2, comes to convoy three separate ideas: (a) that the substance referred to is a compound of calcium and carbon only, and that it is therefore a carbide of calcium; (b) that it is composed of one chemical part or atom of calcium and two atoms of carbon; and (c) that it contains 40 parts by weight of calcium combined with twice twelve, or 24, parts of carbon. It follows from (c) that the weight of one chemical part, now termed a molecule as the substance is a compound, of calcium carbide is (40 + 2 x 12) = 64. By identical methods of calculation it will be found that the weight of one molecule of water is 18; that of acetylene, 26; and that of lime, 56. The general equation (1) given above, therefore, states in chemical shorthand that 64 parts by weight of calcium carbide react with 18 parts of water to give 26 parts by weight of acetylene and 56 parts of lime; and it is very important to observe that just as there are the same number of chemical parts, viz., 2, on each side, so there are the same number of parts by weight, for 64 + 18 = 56 + 26 = 82. Put into other words equation (1) shows that if 64 grammes, lb., or cwts. of calcium carbide are treated with 18 grammes, lb., or cwts. of water, the whole mass will be converted into acetylene and lime, and the residue will not contain any unaltered calcium carbide or any water; whence it may be inferred, as is the fact, that if the weights of carbide and water originally taken do not stand to one another in the ratio 64 : 18, both substances cannot be entirely decomposed, but a certain quantity of the one which was in excess will be left unattacked, and that quantity will be in exact accordance with the amount of the said excess--indifferently whether the superabundant substance be carbide or water.
Hitherto, for the sake of simplicity, the by-product in the preparation of acetylene has been described as calcium oxide or quicklime. It is, however, one of the leading characteristics of this body to be hygroscopic, or greedy of moisture; so that if it is brought into the presence of water, either in the form of liquid or as vapour, it immediately combines therewith to yield calcium hydroxide, or slaked lime, whose chemical formula is Ca(OH)_2. Accordingly, in actual practice, when calcium carbide is mixed with an excess of water, a secondary reaction takes place over and above that indicated by equation (1), the quicklime produced combining with one chemical part or molecule of water, thus--