VALUATION OF MANURES. 375 



at home, and within distances requiring but moderate expense for 

 transportation. 



In the light of such facts you will easily perceive the diflSculty 

 of fixing a definite price upon an article, which, although always 

 the same substance, is yet found in forms and with accompaniments 

 which affect its value to a very considerable degree. The best 

 which can be done in fixing a legal rate is to settle upon what may 

 be a fair average. 



In 1858, Prof. S. W. Johnson decided upon four and a half 

 cents per pound for insoluble phosphoric acid. This corresponds 

 very nearly with the valuation of most European chemists. Since 

 that time the increased supply has kept so even pace with the 

 increased demand that the market value of substances containing 

 it, (excepting raw bone,) have changed very little, if at all, except 

 for the difference between gold and currency. Consequently, if we 

 add to Prof. Johnson's price of 4^ cents, say one-third, for the 

 premium on gold, we have six cents per pound — and this is con- 

 sidered a fair price at the present time, and is that named in the 

 statute. 



Let us next inquire what is a fair valuation of soluble phosphoric 

 acid. Soluble phosphoric acid, whether designed for manure or for 

 other uses, is always obtained by the action of sulphuric acid upon 

 ordinary phosphates. Phosphate of lime (which is the phosphate 

 most frequently met with in nature, and which commonly goes 

 under the name of "bone earth," no matter whether actually obtain- 

 ed from bone, or from minerals, or from fossils), consists of one part 

 of phosphoric acid combined with three parts of lime. This is in- 

 soluble. But another combination of the same constituents exists, 

 which is very easily soluble, and this consists of one part of each. 

 The method uniformly practiced to obtain this soluble phosphate 

 is, to add to ordinary phosphate sufficient sulphuric acid (oil of 

 vitriol, as it is commonly called), to take away from the phosphoric 

 acid two of its three parts of lime — leaving the phosphoric acid 

 combined with one part of lime. This is readily effected, inasmuch 

 as the sulphuric acid has a stronger affinity (to use the common 

 term), for the lime than phosphoric acid has. When thus treated 

 we have a mixture of sulphate of lime and soluble phosphate as 

 the product. 



It follows, therefore, that the cost of manufacturing soluble phos- 

 phoric acid depends largely on the price of sulphuric acid, and as 

 the sulphur, the nitre, the labor and the apparatus, which go to 



