14 ON THE REDUCTION OF IRON BY HYDROGEN. 
capacity of the tube ; a gauze is then placed over the top— 
a few turns of annealed wire drawn around the whole, and 
the mass slipped carefully into the tube until it approaches 
within half an inch of the opposite end. 
The iron stopper h is now covered externally with a soft 
lute composed of fine clay, lime and white of egg, and water, 
and inserted into its place about a line beyond the edge of 
the tube, and a few tacks driven between the stopper and 
tnbe to render it immovable ; the whole end is then carefully 
luted and dried ; as the luting cracks by drying, it is best 
to cover it finally with a mixture of clay and lime and 
water, with one-twentieth of borax, which forms a fusible 
coating. I have been thus minute in reference to the luting 
because of the inconvenience first experienced by the hy- 
drogen issuing from the joint whilst in the fire, to the cessa- 
tion of its escape at the terminal point of the apparatus thus 
depriving the operator of the only means of judging of the 
progress of the reduction. 
The filled tube is now inserted in the furnace, its re- 
spective ends connected with the other apparatus, the 
joints well cemented with the lute before described, and 
all is ready to proceed : — the stopcock of the gasometer is 
opened, and the gas suffered to flow through the apparatus 
until all the air is displaced, the fire is then built around the 
tube with charcoal and anthracite, and as soon as the tube 
arrives at a dull red heat the gas is allowed to flow in ra- 
pidly. The success of the operation depends on the regu- 
lation of the heat more than on any other feature in it. If 
too little, the deoxidation is not accomplished ; if too great, 
as a bright cherry red heat, the particles are conglomerated or 
welded together, and the quality of the product injured. A 
dull red heat, regularly kept up throughout the whole length 
of the tube is the best criterion to govern the operator. 
The half-gallon jar in the terminal arrangement should 
be well refrigerated, which condenses the aqueous vapour 
and permits only the excess of hydrogen to escape, and the 
