164 
ON THE COMBUSTION OF GAS FOB ECONOMIC PURPOSES 
copper, etc., as colouring agents. I have frequently kept small quantities of 
these compounds in a dry place for 12 months. I have had them almost in a 
paste from damp, and have restored the latter to its original efficiency by drying, 
without any tendency to ignition; and, on the other hand, I had spontaneous 
ignition in less than six hours. 
The cause of this action I believe to be (in most cases) acidity^ either of the 
sulphur or of some other ingredient used. It is well known that most of the 
flour of sulphur, as met with in commerce, has a slightly acid taste. This 
acidity has been attributed to atmospheric oxidation from long exposure, but 
more probably is caused by partial combustion during sublimation. Now, 
supposing sulphur containing a slight trace of sulphuric acid to be mixed with 
chlorate of potash, it will liberate a corresponding quantity of chloric acid : this 
at once oxidizes more sulphur, and so the mutual reaction goes on until tlie 
mixture ignites. But we have also nitric acid in combination, and the nearly 
anhydrous vapours of these two acids will sufficiently account for the heating 
and ignition of the compounds in which they are evolved. 
I have been informed by practical pyrotechnists that they never use sublimed 
sulphur, but buy it in roll and powder it for use as wanted; and I believe that 
latterly the sulphur has been superseded for indoor uses by a mixture of shellac 
and resin. This has the merit of comparative safety, although the brilliance of 
the colours will not bear comparison with those formula} where sulpliur is used. 
The above remarks do not apply to a most dangerous compound for purple 
fire, the formula for which I gave in the first-mentioned paper. This contains 
chlorate of potash, sulphur, nitrate of strontia, and anhydrous sulphate of copper. 
Although the colour is exceedingly beautiful, yet I can enumerate five deaths 
from explosions, in addition to other cases not fatal, v/here this formula was in 
use; and I have had it ignite four times in my own experiments ; in this case 
no acidity of the ingredients seems requisite. 
I am disposed to attribute the ignition, first, to the anhydrous sulphate of 
copper attracting moisture from the air, next to double decomposition of the 
copper salt and chlorate, ultimately forming chloride of copper (possibly dichlo¬ 
ride) with evolution of chloric oxides or chloric acid. I have substituted black 
oxide of copper for the sulphate, and the mixture has not shown any tendency 
to ignition. 
I believe that disaster may be frequently averted by first mixing a few drachms 
of the ingredients in a mortar. If the ingredients are no smell should be 
perceived; but if acid be contained either in the sulphur or other ingredients, 
a peculiar, somewhat ozonic odour will arise, which may be considered as in¬ 
dicative of danger. 
ON THE COMBUSTION OF GAS FOR ECONOMIC PURPOSES. 
BY HENRY LETHEBY, ESQ., M.B. 
[X Lecture delivered before the British Association of Gas Managers^ at St. Martin’s 
Hall, London, on Wednesday, May 23, 1866.] 
At the close of the last lecture which I had the honour of delivering to this Associa¬ 
tion at the meeting in Birmingham,* I referred very briefly to the general phenomena 
of gaseous combustion, and to the principles of the economic use of coal gas. It was 
my intention, indeed, to have entered fully into this matter; but so much time was oc¬ 
cupied in the examination of the chemical and physical properties of the most important 
constituents of coal gas, that little was left for the consideration of this part of our sub¬ 
ject. I have therefore been requested to make it the especial subject-matter of this 
* See pp. 281 and 326 of the last volume of this Journal. 
