84 ON RENDERING SUBSTANCES INCOMBUSTIBLE. 
air ; but this does not seem to be the case, as it falls after 
a time to a powder. 
It struck me that the mode of preventing combustion was 
% not by protecting the wood from the fire merely, as heat 
must cause combustible gases to rise from wood, whether 
there be incombustible substances mixed with it or not, and 
these gases will force their way to the surface where there 
is no longer any preventive to burning. My object then 
was to find a substance which would render the wood 
unfit to burn, and would cause it to give out gases which 
would not burn ; so that whilst the wood itself was being 
preserved, except where in contact with the fire, the gases 
would assist in extinguishing the fire. 
I first tried phosphate of magnesia and ammonia, think- 
ing the ammonia given out would be of use in extinguish- 
ing the fire ; but this was of no value, as a piece of calico 
required to be made quite stiff with it before it was render- 
ed incombustible. The calico was prepared by dipping it 
in a solution of phosphate of magnesia in muriatic acid 
and then in ammonia. It seemed to me that the earthly 
salts are of little use for the purpose required, and that the 
amount of solid matter incapable of evaporation left on the 
cloth, assists in a very small degree. 
Sulphuric acid, however, seemed to present the most 
promising characteristics of a substance incapable of burn- 
ing, and of acting so strongly on vegetable substances as to 
make them incapable of burning. Sulphuric acid itself is a 
body perfectly burnt, or we may say overburnt, having an 
atom of oxygen given to it by artificial means, so to speak, 
which atom is difficult to separate, and therefore not resem- 
bling the oxygen of many highly oxydized bodies. It 
requires a high degree of heat to raise it to vapor ; and the 
vapor formed is sluggish and heavy, remaining long where 
formed, and quenching flame wherever it is. It destroys 
the texture of wood also and other vegetable substances, 
causing them to give out after a time gases which do not 
