520 FURNACE 



burn at tho orifice in tho clay plate, n, it must at once bo extinguished and relighted, 

 otherwise some of the gaseous mixture will pass into the chimney unburned, arid 

 subsequently ignite and cause an explosion. A large flame now issues from tho top 

 orifice, and is white if too much gas is on, and chiefly violet or red with tho proper 

 quantity ; it should now be coarsely adjusted until these appearances are presented. 

 The annular plug, c, should now bo inserted, which will compel it to pass down- 

 wards to the chimney, and as soon ns the small remaining flame, now issuing, dis- 

 appears or nearly disappears, as it will in a few seconds, the small stopper, D, should 

 also be inserted. In lieu of this, the large flame may be deflected against the 

 chimney by means of a piece of sheet-iron until it withdraws inwards as before men- 

 tioned ; two plugs may then be reinserted. The gas-tap may now bo partly adjusted. 

 The crucible should bo placed in the furnace after the act of lighting the gas, 

 but not immediately after if tho furnace is cold, or explosions may occur by unburncd 

 gaseous mixture passing the crucible into the chimney and igniting afterwards. 

 After about five minutes the gas should be slowly adjusted, until a sound is heard 

 inside like a series of small explosions. This sound is sometimes not very distinct, 

 especially at high temperatures, and therefore requires a little experience in tho 

 use of the furnace in order to be detected. It is, however, a chief guide in de- 

 termining the proper amount of gas, and should therefore be carefully studied. 

 To assist in adjusting the gas, it will be found very useful to place a small piece of 

 looking-glass beneath the tube, L, and to adjust the gas-tap until the flame between 

 the burner and crucible appears wholly violet or slightly white ; but this test is 

 liable to fallacy if employed when the gas is just lighted, because the coldness of the 

 part makes tho flame much whiter than it otherwise should be. It is also fallacious, 

 the flame appearing whiter than it really is when the crucible is very hot. It is, 

 however, of great assistance, especially at intermediate temperatures. A rough 

 deposit upon the outer edge of the crucible indicates an excess of gas : the deposit is 

 carbon. Less gas is required with a crucible in the furnace than without one ; also 

 less is required when the small hole at the top of the furnace is open than when it is 

 closed ; and less is also required when the furnace is cold than after it has been 

 lighted some time, because the draught gradually increases and draws in more air. 

 After having accurately adjusted the gas, no further attention to the furnace is 

 requisite. 



Having once found the proper adjustment of gas under certain known conditions, 

 it is well to notice the position of the index-pointer, N, in order to be able at once to 

 adjust it to about the right point on other occasions. 



Under ordinary circumstances, during daylight, I have found it best to set the gas 

 nearly full on at first, and fully on at about five minutes afterwards, when tho 

 draught has become more powerful ; but during twilight, when tho supply of gas 

 from the gas-works is more free, I have set the index-pointer at the numbers 2! or ;i. 

 The gas should be supplied by a pipe of not less than three-eighths of an inch bore, 

 with a main pipe of half an inch ; but all depends upon the pressure of gas at tho 

 particular locality, which is very variable. The consumption of gas varies from 

 thirty to forty cubic feet per hour, the value of which is about twopence. 



The top of the chimney should be placed in a position where tho products of com- 

 bustion can pass freely away. If it is placed in an opening or pipe leading to 

 another chimney, care must be taken not to have the draught too powerful, otherwise 

 the heat will be drawn more into the chimney, and tho supply of gas in tho daytime 

 may be found rather deficient. The furnace will act satisfactorily, though less 

 powerfully, with tho chimney standing in an open room without any special outlet 

 for tho products of combustion, provided the full height (6 feet) of chimney is em- 

 ployed. Under other circumstances I have generally used a chimney 4 or 5 feet 

 high. ^ 



This furnace will produce what is generally called ' a white heat; ' it will readily 

 melt half a pound of copper, or six ounces of cast iron. I have melted those quan- 

 tities in it : it will melt as large a quantity of those substances as the largest sized 

 crucible that can bo introduced into it will contain, sufficient space being reserved 

 around tho crucible for draught. It requires from twenty to thirty minutes to 

 acquire its highest temperature, and then the entrance part of tJic chimney r.i7/////.s- a 

 faint red heat in daylight. If it exhibits much more than this, the draught is too 

 powerful, and if less, there is not sufficient gas. 



When the small hole, D, is open, some air is drawn in that way, and less air ] 

 up with the gas through the tube o, but tin- cold air docs not much diminish tho 

 temperature of the crucible, because it combines with the excess of gas now passing 

 over tho edge of the inner cylinder; it, however, renders the flame round tho 

 crucible white by deficiency of air, and this should be partly corrected by lessening 

 the gas. An excess of cither gas or air renders the surface of melted C 

 dull. 



