370 



GAS LIGHT. 



sel through the pipe J, whose top reaches nearly 

 to the lid of the purifier. This pipe passes out at the 

 bottom of the vessel, and opens into a cylindrical 

 vessel, K, containing liquid lime, or lime cream, as 

 it is usually called. In this vessel, tlie purifying pro- 

 cess is completed, and, in order that the gas may be 

 brought into contact with as much of the cream as 

 possiole, the liquid is kept in agitation by means of 

 floats or oars revolving on an axis, which is put in 

 motion by the handle M ; which, in small establish- 

 ments, is turned occasionally by the attendant of the 

 apparatus ; but, in large gas manufactories, it is 

 kept in constant motion by machinery. The lime 

 cream is mixed in a box, ( *, and is admitted, when 

 inquired, into the purifier, through a pipe furnished 

 with a stop-cock, as seen in the engraving. In the 

 liquid purifier there are three division plates, placed 

 horizontally, and perforated with holes at one end, 

 the perforated ends being alternate, so that, when 

 the gas passes up through one plate, it must traverse 

 across the vessel before it can rise through the 

 second, and so with the third, after passing through 

 which the purified gas ascends through the pipe P, 

 into the gasometer R. 



All this purifying process is instituted with a view, 

 as before stated, of separating carbonic acid and 

 sulphuretted hydrogen from the gas, the presence of 

 either of which is highly detrimental to its illuminating 

 power. The attendant on the apparatus ought fre- 

 quently to test the purity of his gas, in order to de- 

 termine whether the lime ought to be changed, or the 

 retorts recharged. The presence of sulphuretted hy- 

 drogen may be easily detected by allowing the gas 

 to play upon a piece of white paper, moistened with 

 a solution of acetate of lead (sugar of lead), as the 

 paper will soon become black ; and the presence of 

 carbonic acid gas may be detected, by shaking some 

 of the gas with pure lime water, in a phial ; the mix- 

 ture will soon become turbid, by the action of the 

 carbonic acid, if it be present. 



The gasometer, into the bottom of which the pipe 

 R passes, is a sort of storehouse for the gas. It 

 consists of a hollow cylindrical vessel, usually made 

 of cast metal plates, open at one end, and placed 

 upon its open end in a cistern of water rather larger 

 in dimensions than itself. The principles of its 

 construction are precisely the same as those of the 

 gasometer employed in pneumatic chemistry. The 

 best proportions of the gasometer are, that the 

 depth should be equal to twice the diameter, and 

 the capacity must be regulated by the quantity of 

 gas required for consumption. The greatest care 

 should be taken tliat it be made strong and tight, 

 by being well rivetted, and braced with malle- 

 able iron stays. The plates are made to overtop 

 each other, similar to those of steam engine boilers, 

 slips of canvas, besmeared with white lead and oil, 

 being interposed, to ensure perfect tightness. The 

 cistern or tank, R R, is some times made of mason 

 work, and not unfrequently of cast iron plates. The 

 pipe P from the purifier passes up through the bot- 

 tom of the tank, and terminates in an opening above 

 the surface of the water, with which it is nearly 

 filled ; this is called the induction pipe. There is 

 another pipe of similar dimensions also opening into 

 the tank, but led out in a different direction from the 

 induction pipe, for the purpose of conveying the gas 

 to the place where it is to be consumed. 



As the weight of a gasometer is very consider- 

 able, it is necessary to diminish it, so that the pressure 

 of the gas which enters from the purifier may cause 

 it to rise in the tank. This end is accomplished by 

 a counterweight, S, suspended by a chain passing over 

 the pulleys T T an arrangement which will be at 

 once understood by an inspection of the plate. The 



regulation of the weight of the counterpoise is a 

 matter of some nicety, as it is evident, from the prin- 

 ciple of hydrostatics, that the weight of the gasome- 

 ter will increase as it rises out of the tank. In order 

 to compensate for this variation of weight of the 

 gasometer, the differences of its weights in and out 

 of the water must first be condescended upon. Now 

 it has been found that cast iron loses about one- 

 seventh of its weight when immersed in water ; and it 

 is clear that as the gasometer ascends, the suspen- 

 sion chain, S T, is lengthened, or, to speak more cor- 

 rectly, that portion of it which adds to the effect of 

 the counterweight is increased ; and, therefore, if so 

 much of the chain as is equal in length to the height 

 of the greatest elevation of the gasometer, be made 

 equal in weight to half of the weight lost by the 

 gasometer by immersion, then will the variation of its 

 weight be compensated for, and its pressure upon the 

 gas rendered uniform. 



When the gas in the gasometer is to be expelled 

 for use, a certain pressure must be exerted. This is 

 effected by diminishing the counterweight, by a 

 quantity which is usually estimated as equivalent to 

 the pressure of a certain depth of water : thus, if it 

 be required that the gas should issue from the gaso- 

 meter with a force equal to the pressure of one incli 

 of water, we have only to find the area of the bottom 

 of the gasometer, and see what pressure would be 

 exerted by a column of water with this base and a 

 height of one inch. Now if the area of the cross 

 section of the gasometer be 1000 inches, since the 

 pressure of one inch of water is -036 of a pound, the 

 pressure will be 1000, multiplied by -036, that is 

 36 Ibs. 



The consumpt of gas in large towns is by no means 

 regular, and therefore the effect of the constant and 

 equable pressure of the gasometer will in a great 

 measure be destroyed by the unequal expenditure of 

 gas. The contrivance employed for overcoming this 

 is at once simple and ingenious. The pipe A , by which 

 the gas issues from the gasometer, is led into a small 

 vessel, furnished with a conical valve, C, which opens 

 into a vertical pipe leading into a vessel inverted in 



water, resembling a gasometer. The pipe B leads 

 the gas off for the supply of the jets. If the consumpt 

 of gas become greater than the supply, then will the 

 pressure on the bottom of the valve C be increased 

 by the gas from the pipe A, that on the upper side 

 from the gas in the pipe B being diminished ; the 

 valve C will therefore be raised a little, and conse- 

 quently more gas will issue out of the gasometer. 

 Again, if a number of the jets be taken off, and the 

 consumpt become less, then will the supply exceed 

 the demand, and the pressure of the gas in the pipe B 

 will exceed that of the gas in the pipe A, when, 

 in consequence, the valve C will have a tendency to 

 close, and so allow less gas to issue out of the gaso- 

 meter. 



Before the gas is consumed, it is frequently passed 

 through an instrument called a meter, in order to 

 ascertain the number of cubic feet which are used 

 in a given time, or in a particular place. The an. 

 nexed wood cut will show the construction of this 

 instrument : 



