354 SPONTANEOUS COMBUSTION OF WOOD ASHES. 



naturally rich in vegetable matter, or be mixed from year to year with a 

 sufficient quantity of animal or vegetable manure. 



In so far as the immediate effect of wood ashes is dependent upon the 

 soluble saline matter they contain, their effect may be imitated by a 

 mixture of crude potash with carbonate and sulphate of soda, and a lit- 

 tle common salt. The wood ash of this country contains only about 

 one-fifteenth of its weight of soluble matter (Bishop Watson), so that 

 the following quantity of such a mixture would be nearly equal in effi- 

 cacy to the saline matter of one ton of wood ash. 



Crude of Potash 60 lbs. at a cost of 15s, 



Crsytallized Carbonate of Soda . . 60 " " " 7s. 



Sulphate of Soda 20 " ) „ .« Oc 



Common Salt 20 " $ "'^• 



160 24s. 



Where the wood ash costs only a shilling a bushel (or 662 a ton), it 

 would obviously be more economical to employ this mixture, were the 

 efficacy of wood ashes dependent solely upon the soluble saline matter 

 they are capable of yielding on the first washing with water. But they 

 contain also a greater or less quantity of imperfectly burned carbonace- 

 ous matter, the effect of which upon vegetation cannot be precisely 

 estimated, and a large proportion — nine-tenths, perhaps, of their whole 

 weight — of insoluble carbonates, silicates, and phosphates of potash, 

 lime, and magnesia, which are known more permanently to influence 

 the fertility of the land to which they are applied.* 



c. Washed or lixiviated wood-ashes. — fn countries where wood ashes 

 are washed for the manufacture of the pot and pearl ash of commerce 



' Some discussion has lately arisen in America (Silliman's Journal, xlii. p. 165, and 

 xliii. p. 80), in regard to the fact, in itself sufficiently interesting, that wood ashes, when 

 thrown together in heap', not unfrequently take fire, becoming red hot throTjghout their 

 whole mass, and sometimes occasioning serious accidents. Such ashes always contain a 

 quantity of minutely divided carbonaceous matter, which, like the impalpable charcoal 

 powder of the gunpowder manufactories, may have the property of absorbing much air 

 mto its pores, and of thus undergoing-a spontaneous elevation of temperature. I throw it 

 out, however, as a more probable conjecture, (hat during the combustion of the wood a 

 portion of the potash has been decomposed by the charcoal, and converted into potassium 

 (potash consisting of potassium and oxygen, p. 1S7. When exposed to tHfe air and to 

 moisture this potassium gradually absorbs oxygen and spontaneously burns, again form- 

 ing potash. That such a decomposition may take place where wood or other vegetable 

 matter is burned with little access of air will, readily be granted, but it is not so obvious 

 that it can take place in an open fire. But even in an open fire, or in an open capsule, par- 

 ticles of potassium may remain in the pores of the unburned charcoal, or more frequently 

 may be covered over with a glaze of melted potash, by which further combustion will be 

 prevented. That this really does happen, any one must have satisfied himself who has 

 been in the habit of burning vegetable substances for the purpose of determining the pro- 

 portion of ash they leave. The glaze of melted alkaline matter often renders the com- 

 plete combustion a very difficult and tedious matter. That potassium is formed during this 

 process is rendered further probable by the observation that the quantity of potash ob- 

 tained from wood or other vegetable ash is less when the wood has been burned at a high 

 than a low temperature. The potassium, which is volatile, may have been dissipated in 

 vapour. 



It is probable that a spontaneous combustion similar to that observed in America may 

 occasionally take place in the heaps of ashes left f«n stand upon our fields af>er paring and 

 burning— and hence probably has arisen the practical rule, to sprea<l the ashes as soon as 

 possible after the burning is finished. If allowed to remain, they are .'aid "/o take hold of 

 the land," and when it is of clay, to burn it into brick. An instance of such combustion is 

 mentioned as having occurred at Chatteris, in the Isle of Ely, where an entire common 

 was burned 16 or 18 inches deep, down to the very gravel.— See British Husbandry, II. j 

 p. 350. 



