PRINCIPLES OF MANURING 121 



with sulphate of ammonia at 12 per ton, 

 that is 11s. 8d. per unit of nitrogen, we find, 

 by multiplying 15'6 by 11s. 8d., that the 

 corresponding price for nitrate of soda is 

 9, 2s. per ton. 



It was at one time common to speak of 

 nitrogen and ammonia almost as if they were 

 interchangeable terms. Merchants, too, often 

 stated the percentage of their manures both 

 in terms of nitrogen and ammonia, and 

 sometimes even in terms of ammonia only. 

 This was frequently done in order to attract 

 the eye of a possible buyer with a larger 

 figure, the relationship between nitrogen and 

 ammonia being as 14 is to 17. In other 

 words, 17 Ib. of ammonia consist of 14 Ib. 

 of nitrogen chemically combined with 3 Ib. 

 of hydrogen ; so that, for instance, the 

 statement that a manure holds 8'5 per cent, 

 of ammonia, is no better than saying that it 

 holds 7 per cent of nitrogen. It is now, 

 however, illegal to omit to state the per- 

 centage of nitrogen, if any, in the invoice 

 that must accompany the delivery of a 

 manure. 



Valuation by units is a rapid and useful 

 method by which farmers may estimate 

 whether manures that are offered to them 

 are cheap or dear. Some Agricultural 



