ON THE REDUCTION OF IRON BY HYDROGEN. 
15 
relative volume of the gas bubbles passing through the 
water in the washing jar, and that at the terminal tube, will 
^ive the best index of the consumption of the gas. Besides, 
the amount of water condensed is also a means of judging. 
Assuming the sub-carbonate of the Pharmacopoeia to be in 
a hydrated condition, about 14 oz. of water should be 
evolved from 32 oz. of the sub-carbonate. ^When the latter 
quantity of material is operated on, the process is occupied 
from 5 to 8 hours. The current of gas should be kept up 
after the removal of the fire until the apparatus cools to the 
ordinary temperature, but in small quantity. This is neces- 
sary to exclude the air, because the reduced iron if exposed 
to the atmosphere whilst hot instantly takes fire and is con- 
verted into sesquioxide. 
When cool, the stopper and luting is carefully removed, 
and the gauze drawn out on to a sheet of paper. What was 
a solid mass at first is now a series of pieces of wire gauze 
covered with the iron in pulverulent slightly cohering 
masses of an iron grey colour and occupying about one- 
eighth of the former space. The pieces of gauze are care- 
fully separated and examined. If any part of the iron is 
black instead of iron grey, it must be separated from the rest, 
as it is not fully deoxidized, and is very insoluble in diluted 
sulphuric acid. When the iron is perfectly reduced and 
pulverulent, it effervesces like a carbonate when thrown into 
dilute acids; this, together with its light colour, and freedom 
from taste, are the best criteria for judging of its quality. 
When a small mass is struck with a smooth hammer on an 
anvil, it is compressed into a scale with a brilliant metallic 
lustre. 
Some are impressed with the idea that this operation is 
attended with much danger, which is not true if the direc- 
tions above given are followed. On two occasions I have 
had explosions resulting in the entire destruction of the jar 
containing lime, but this was before I was aware of the ne- 
cessity of using so large an amount of zinc. In renewing 
