313 



GAS, MANUFACTURE OF. 



GASES, COLLECTION OF. 



Burner's meter causes a loss of pressure from its friction equivalent to 

 about one-tenth of an inch. [See METERS, WATER AND GAS.] 



5. The accessory buildings required for a gas factory, in addition to 

 those required for the station meter and the governor, are merely 

 those required for storing the coal, for receiving the fresh and the 

 spent lime, and the coke and tar for sale ; and occasionally some rooms 

 for experiments upon the quality, and the chemical composition, of 

 the gas ; though in small works these rooms are but rarely required. 

 If the works should be placed at any distance from the source of supply 

 " of fuel, the size of the coal sheds should be made sufficient to contain 

 about three months' consumption. The coke shed need not be large, 

 for it is decidedly the interest of the maker to sell it at once ; and the 

 stock of tar should at all times be kept as low as possible, on account of 

 its inflammable nature. The lime-sheds also do not require any great 

 capacity, but they should be weather-proof. 



The dimensions of the chimney must be calculated according to the 

 ordinary laws of the motion of heated elastic fluids, so aa to ensure 

 an efficient draught under the retorts without wasting fuel. Lofty 

 chimneys are usually mistakes in gas-works, and it is a common maxim 

 amongst practical engineers, that the profits are in the inverse ratio of 

 the height of a chimney. In fact, in an open country a height of 45 

 or 50 feet is sufficient for a chimney serving for four beds of five 

 retorts each. And here, perhaps, it may be as well to observe that it 

 is in executing the foundations for the retort beds, the chimney stalk, 

 and the gas holder, that the greatest difficulties arise in practice ; and 

 they require all the attention and the skill of the engineer. 



In most country factories residences are provided for the workmen 

 who are constantly employed ; a small office and store room is con- 

 structed ; and rooms are provided to receive a stock of meters, and of 

 fittings ; for the country companies in many cases are obliged to retain 

 this branch of trade in their own hands, in order to secure its proper 

 execution. Sometimes a board-room, and a meeting-room, are also 

 pruvided. 



The operation of testing the quality of a gas, with respect to its 

 illuminating properties, is an operation of so delicate and so important 

 a nature, that it may be desirable to dwell a little upon this part of 

 the science of Photometry, at least so far as may be necessary to 

 explain its ordinary application in commercial affairs. 



The principles upon which the photometers in general use are con- 

 structed, are based upon the law that " the power of a luminous body 

 to light a given space is directly proportional to the intensity of the 

 light itself, and inversely proportional to the square of the distance of 

 the point of observation from it ; " and its application is obtained by 

 causing the two sources of light it may be desired to compare, to shine 

 upon the respective sides of a thin and partially translucent body. 

 When the illumination of the exposed surfaces of the latter is equal, 

 that is to say, when neither of these faces possesses a greater degree of 

 luminosity than the other, the illuminating powers of the sources of 

 light with respect to the interposed body are said to balance one 

 another. If then one of the sources of light be a lamp, or candle, 

 possessing known illuminating properties, it is easy to calculate the 

 relative value of the other, in terms of the power of the standard, 

 according to their respective distances from the interposed object. 

 The difficulty, of course, lies in the selection of an invariable standard 

 for this purpose, and the choice of gas engineers varies between wax 

 and spermaceti candles, or the Carcel lamp. In England, the standard 

 usually adopted is the spermaceti candle, burning 120 grains per hour ; 

 in France, the standard is frequently the Carcel lamp, burning 42 

 grammes (2'4 oz.) of rape oil per hour; and when the light of either 

 of those illuminating bodies is to be compared with gas, the latter 

 must be admitted in certain definite quantities (usually about 5 cubic 

 feet per hour), and under a carefully regulated pressure. 



The thin partially illuminated screen above referred to, in the 

 Bunsen photometer (the one usually adopted with various modifications 

 in England), presents a surface which is white, but nearly opaque on 

 the circumference, and perfectly translucent in the centre. The pre- 

 paration of this disk is a matter of great delicacy, for the translucent 

 part should differ as little as possible in colour from the rest of the 

 surface, when the illumination of its sides is equal. Dr. Fyfe, and the 

 majority of English makers of photometers, use cream-coloured writing- 

 paper for the screen, and they mark the translucent centre covering 

 it with a solution of spermaceti in oil of naphtha equally diffused over 

 its surface ; the diameter of this translucent centre is usually about 

 the same as that of a half-crown piece. In the photometers made by 

 the late Alexander Wright and his successors, the screen was placed 

 in the middle of a cylindrical shade, blackened on the inside : and it 

 worked upon a bar graduated so as to express at once the value in 

 candles and decimal parts of candles, of the legal standard, of the light 

 experimented upon. Great care ought to be taken to exclude any 

 extraneous rays of light derived from reflection from the sides of the 

 ro.iin, or from the surface of the graduated bar ; so that, in fact, none 

 but the direct rays from the respective sources of light should be 

 allowed to impinge upon the screen. The length of the bar in Wright's 

 l>h '("meter is about 100 inches; in Church & Mann's photometer it is 

 considerably less ; and, indeed, for some reasons the latter instrument 

 seems to be the most convenient one of its kind. 



It is curious that notwithstanding the importance of the operations 

 of photometry, in many cases of disputed contract, no attempt has been 



made to ascertain the limits of what M. Quetelet calls " the personal 

 equation of observation " in them. The powers of every individual 

 to appreciate degrees of luminosity differ, more or less ; and the singular 

 discrepancies in the results of the recorded observations on this subject 

 may be to some extent accounted for on this ground. 



A series of tables of the illuminating powers of the gas obtained from 

 the coals generally used in England, will be found in the new edition 

 of Clegg ; a book which, by the way, entirely supersedes all other works 

 upon the subject. In the same work there are some equally interesting 

 tables of the composition of the various coals themselves, and a valuable 

 chapter xipon the modes of dealing with the residual products of the 

 manufacture of gas. The reader is, therefore, referred to Clegg's 

 ' Practical Treatise on the Manufacture of Coal Gas,' 1859 ; to 

 D'Hurcourt's ' Trait(5 de 1'Eclairage au Gaz,' 1845 ; Peekston, ' On the 

 Theory and Practice of Gas Lighting,' 1823 ; and Accum's ' Practical 

 Treatise on Gas Light,' 1815. In the ' Journal of Gas Lighting,' a vast 

 amount of useful practical and theoretical information may be found 

 on this subject ; and likewise in the ' Practical Mechanics' Journal.' 

 The latter, particularly, contains some elaborate articles on the various 

 systems introduced by Mr. Bowyer, of St. Neot's, for the purpose of 

 applying gas on a suitable scale for the economical lighting of small 

 towns, and even of private houses. Some notion of the national im- 

 portance of the gas industry may be formed from the fact that there 

 are in existence in the United Kingdoms about 1000 gas companies, 

 and that it is able to support special journals, not only in England, 

 but also in France, Germany, and the United States. 



GASES, COLLECTION OF. The collection and preservation of 

 gases form a class of chemical operations so essentially distinct from 

 those employed in manipulations with the other two forms of matter, 

 that it will be useful to describe them in a separate article. 



The chief difficulty to be overcome in manipulations with gases is 

 their diffusibility or tendency to mix with atmospheric air, a property 

 which gases possess to such an extent that comparatively few substances 

 are capable of entirely resisting the passage of gaseous matter through 

 their pores. This diffusibility renders the collection of gases by dis- 

 placing one gas, contained in any vessel, by means of another, always 

 an imperfect and frequently an impracticable operation ; nevertheless, 

 where the two gases differ considerably in specific gravity, and when 

 absolute purity is not required, this process may sometimes be resorted 

 to with advantage. 



Collection of yascs by gaseous displacement. In this process the gas 

 displaced is always atmospheric air, and in order to apply it success- 

 fully it is necessary that the gas should be generated in a copious 

 stream, and it is also desirable that its specific gravity should differ 

 considerably from that of the atmosphere. Tall and narrow glass 

 cylinders or bottles are best adapted for this purpose. If the gas to 

 be collected be heavier than air, the jar in which it is received must be 

 placed with its mouth upwards, and the tube conveying the gas must 

 terminate at or near the bottom of the jar, as in fig. 1. If, on the 

 other hand, the gas be lighter than air, the jar must be inverted, and 



Fig. 1. 



Fig. 2. 



Fig. 3. 



the gas delivery tube must reach as before to the closed end of the 

 cylinder, as in Jiy. 2. 



A modification of this method is frequently employed in the collec- 

 tion of gases for analyses, where a high degree of purity is indispensable. 

 The gas-delivory tube is connected with a glass tube or series of tubes 

 drawn out at both ends, as shown in Jig. 3, and the current is con- 

 tinued until the last traces of air are expelled, an operation which is 

 facilitated by connecting the extremity where the gas is issuing with a 

 long caoutchouc tube, which prevents the retrogression of air by 



