86 ON RENDERING SUBSTANCES INCOMBUSTIBLE. 
as it cannot be applied to cloths to be frequently washed. 
True, it is so cheap that it might be applied every washing 
where there are peculiar dangers ; but if a person was 
standing very near the fire, the ammonia would in part be 
evaporated, and the acid remaining would be enough to 
injure the fabric. There are, however cases, such as cur- 
tains, to which this could not apply, and where it would be 
valuable. 
Sir William Burnet's liquid is chloride of zinc : he uses 
it for preserving wood and canvass, and also for preventing 
fire. I am certainly surprised that more use has not been 
made of it, being as far as I have seen it, so efficient. I 
believe the manner in which the chloride of zinc acts is 
very similar to that of sulphuric acid, destroying the organic 
matter on the approach of heat, and rendering it incombus- 
tible. It can be introduced into wood at a specific gravity 
of 2000, I believe ; sulphate of ammonia cannot easily be 
used above 1200. By heating the solution more may be 
attained. Sulphate of ammonia is cheap and easily pro- 
cured and used, not hurting anything with which it may 
come in contact, and therefore more easily managed in 
households. 
The chloride of zinc is said to unite with the fibre. This 
cannot be said for the sulphate of ammonia. It would nor, 
however, come from the centre of a beam of wood, even if 
immersed in water, as the water enters with great difficulty 
into wood ; and the solution itself cannot be introduced 
without forming a vacuum in the saturated vessel, and so 
removing all the air from the wood. 
The first time I used this solution I found a large quantity 
of mould formed, and indeed it contains all the elements to 
increase its growth. The second time the solution was 
boiled in an iron vessel, and no mould formed on it; on the 
contrary mould was destroyed by it. The sulphate of am- 
monia dissolves iron rapidly, and formes a double salt 
which is deleterious to such growths. I imagined any 
