On Spontaneous Inflammations . %5l 
the carbonaceous matter around it, and which being in a 
state of great tenuity is the more disposed to burn. 
But as many of our common combustible matters con- 
tain sulphuric salts, it may happen that in their combus- 
tion there is sometimes accidentally formed some pyro- 
phoric matter, which remains in the residuum of the com- 
bustion ; especially if the combustible matter is not en- 
tirely consumed, and if a part of it only is reduced to 
charcoal ; which sometimes happens in fire-places where 
the combustibles are not burnt in grates, and where the 
ashes are not separated from the charcoal. There have 
been instances of houses being set on lire by ashes inter- 
mixed with charcoal taken too soon from the hearth and 
deposited in places where they were surrounded by com- 
bustibles, which they set fire to by spontaneous inflam- 
mation. Happily these causes of fires rarely occur ; for 
pyrophorus does not long retain its property of inflaming, 
and it is often decomposed soon after it has been formed, 
without being able to produce that disagreeable event. 
Care, however, ought always to be taken not to put ashes 
newly burnt, and which are still mixed with charcoal, in 
places where they may have a communication with com- 
bustibles. 
The formation of a pyrophoric matter is remarked chief- 
ly in the preparation of common soda, which is obtained 
by the incineration of several marine plants containing a 
great deal of sulphate of soda, and which in burning fur- 
nish always a greater or less quantity of sulphur accor 
ding to the maimer in which the operation is directed. 
The formation of the phosphuret of lime has great ana - 
logy with that of the sulphuret of lime. Though the phos 
phone acid is not found so often in vegetables as the sul 
phuric acid, it however exists in them in much larger 
quantity than has hitherto been supposed ; it is found 
chiefly in the greater part of plants which grow in marshy 
