ON RENDERING SUBSTANCES INCOMBUSTIBLE. 85 
burn, mixed with some which do burn ; but if there be 
enough of acid, forming a mixture which does not burn. 
The wood also cannot be again induced to become combus- 
tible until it be heated to redness, so as to remove all the 
sulphuric acid, leaving only charcoal. 
If sulphuric acid then could be introduced into wood 
just at the time that the fire was going to take place, the 
fire would cease to take place : and this we can do easily 
by saturating the wood with suLphate of ammonia. When 
there is no fire present there is no sulphuric acid present, 
as such ; but as soon as the heat rises, ammonia goes off, 
and sulphuric acid is instantly presented to the wood. The 
ammonia does not come off quite pure, it is mixed with 
nitrogen and sulphurous acid ; and this disengagement of 
gases is of advantage in extinguishing fire ; when the heat 
rises to 536°, the sulphuric acid is then left to act on the wood 
in part and to volatilize in part, and that which I have men- 
tioned takes place. The outside of course would first 
undergo the change, and the inside would be protected by 
the incombustible outer part; if the fire continued to act 
long, the inner layer would undergo a similar change. I 
imagine, then, the acid acts in a double manner ; it makes 
the wood refuse to burn, and it puts out fire. As sulphur- 
ous acid is given off in this process, the action is also simi- 
lar in one point of view to that of sulphur, which has long 
been used for putting out fire in chimneys. 
I have no doubt that a house of wood prepared in this 
manner might have a fire lighted on the wooden floor 
without danger, burning only on the spot to which the 
fire was limited. A ship also would be safe, even if the 
cinders did fall from the grate in stormy weather. 
I know that muriate of ammonia has been used, and that 
it acts very well ; but I think the sulphuric acid is superior, 
the ammonia being merely to keep it innocent ; and other 
volatile base might do. I am sorry, however, that this is 
not perfect ; its solubility in water is a great disadvantage, 
S 
