Experiments on the Gas from Coal. 405 
vered in the last products of gas. On the 
large scale, both these gases continue to be 
evolved throughout the whole operation, 
though in greatly diminished proportion to- 
wards the latter end. Even in the advanced 
stages of large distillations, the presence of 
sulphuretted hydrogen in coal gas may be 
traced by the proper test, though not in a 
quantity that admits of being easily measured. 
The test, which I used for some time, was 
the white oxide of bismuth, for’ which I 
afterwards substituted white lead, ground 
with a little water to the proper consistence, 
and spread by a camel’s hair pencil on a’slip 
of card. This was secured by asmall pair of 
forceps fixed in a cork, by means of which, 
the slip of card could be placed in a jar or 
bottle of the gas, and kept there for some 
time. By experiments on artificial mixtures, 
I found that a cubic inch of sulphuretted hy- 
drogen, diffused through twenty thousand 
cubic inches of common air, distinctly af- 
fected the test, which it changed to a light 
yellowish or straw colour. By mixing sul- 
phuretted hydrogen with various proportions 
of common air, I prepared coloured cards 
of a variety of shades, which served ‘as 
standards of comparison for judging of pro- 
portions of sulpluretted hydrogen in coal 
