Experimental, Analytical, and other purposes, 17 



side it to the chimney, the point of greatest heat being at Q. It 

 is important in using this furnace that the burner is placed quite 

 in the centre of the bottom of the tube ; also that a crucible 

 of not too large nor too small dimensions be selected. The most 

 suitable way of supporting a smaller crucible is by placing it in 

 a larger one that has had its upper parts broken off. If desirable, 

 a little clay luting may be placed round the top edge of the iron 

 casing to exclude air entering between it and the cylinder; also 

 a little thin clay luting upon the part of the bottom of the fur- 

 nace where the inner cylinder rests. 



In lighting the furnace, the plugs C and D are removed, a 

 light held within the opening, and the gas turned on full ; should 

 the flame blow down to the bottom of the tube L on lighting 

 (which, however, rarely occurs unless the furnace is already hot), 

 the gas must be turned off and the bottom end of L momentarily 

 closed whilst lighting the gas as before. Should the flame not 

 burn down to the burner, but only burn at the orifice in the clay 

 plate B, it must at once be extinguished and relighted, otherwise 

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

 and subsequently ignite and cause an explosion. A large flame now 

 issues from the top orifice, and is white if too much gas is on, and 

 violet or red with the proper quantity. This flame generally 

 diminishes and nearly disappears into the body of the furnace in 

 the course of one minute : whether it does or not, the annular 

 plug C should now be inserted, which will compel it to pass down- 

 wards to the chimney \ and as soon as the small remaining flame 

 disappears or nearly disappears, as it will in a few seconds, the 

 small stopper D should also be inserted ; instead of this the 

 large flame may be deflected against the chimney by means of a 

 shoot or bent piece of sheet iron until it withdraws inwards; 

 the plugs C and D may then be reinserted and the gas-tap partly 

 adjusted. The crucible may be placed in the furnace either 

 before or some time after the act of lighting, but not immediately 

 after (if the furnace is cold), or explosions may occur by unburned 

 gaseous mixture passing the crucible into the chimney and igni- 

 ting afterwards. 



After about five minutes the gas should be adjusted until a 

 sound is heard inside like a series of small explosions. This 

 sound is sometimes not very distinct, especially at high tempe- 

 ratures, and therefore requires a little experience in the use of 

 the furnace in order to be detected ; it is, however, a chief guide 

 in determining 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 



Phil, Mag, S. 4. Vol. 26. No. 173. July 1863. C 



