ALKALINE SALTS. 327 



most purposes to fix the ammonia of gas liquor. 

 This may either be done with an acid, with gypsum, 

 or sulphate of iron (792, 794) : a cheap acid like the 

 sulphuric is, generally speaking, the most convenient. 

 Gas liquor resembles putrid urine, inasmuch as it is 

 very rich in ammonia; but it differs from it in not 

 containing the fixed alkaline and earthy salts which 

 that manure does. 



888. As the strength of gas liquor mainly depends 

 on the salts of ammonia which it contains, and as the 

 relative quantity of these salts varies considerably, it 

 is difficult to state the exact quantity of any acid 

 necessary to neutralize the ammonia; the average 

 quantity of strong sulphuric acid requisite for this 

 purpose is from 20 to 25 lbs. for every 100 gallons 

 of gas liquor ; a little excess of acid hardly ever does 

 any harm, as it immediately becomes neutralized by 

 lime, &c., in the soil. 



889. Potash and soda, as has been several times 

 stated, are very commonly found in plants, and con- 

 sequently are important constituents of manure; they 

 both exist in "most soils, though present but in small 

 quantity, and are generally combined with silica and 

 other earthy substances ; these compounds are slowly 

 and gradually decomposed by the carbonic acid of 

 the air (651) ; their decomposition may be artificially 

 assisted by the action of lime (873); the principal 

 salts of the fixed alkalies used as manures are the 

 carbonates, muriates, nitrates, and sulphates. 



890. The ashes of some plants contain a, very large 



