18 Chemical Composition and Commercial Value of 



artificial manures. They are generally distinguished from the 

 Peruvian by the almost entire absence of ammonia and soluble 

 phosphates, by the small quantity of organic matter, and by the 

 large proportion of insoluble phosphates which they contain. On 

 the latter the commercial value of these guanos chiefly depends. 

 They usually contain a considerable quantity of gypsum, and 

 occasionally carbonate of lime besides. If a guano is to be used 

 as a manure without any further preparation, the presence of car- 

 bonate of lime, though it does not add to, does not detract from 

 its value ; but if the guano is to be employed as the basis for 

 superphosphate, or for drying artificial manures containing soluble 

 phosphate, the less carbonate of lime it contains the better. 



If guano containing carbonate of lime is mixed with sulphuric 

 acid, all the carbonate must be saturated with acid before any 

 soluble phosphate can be formed ; and again, if such a guano be 

 added as a drying material to a manure containing soluble phos- 

 phate, the carbonate of lime of the guano, taking up the acid 

 which keeps the phosphate in a soluble state, renders the latter 

 insoluble. In either case more acid is required for the produc- 

 tion of a given amount of soluble phosphate than is wanted if 

 phosphatic materials free from carbonate of lime are employed. 



Saldanha Bay guano, South African, Pedro Keys, Swan 

 Island, Baker Island, Birds' Island guanos are some of the in- 

 ferior phosphatic guanos which may be used either by them- 

 selves, or better after having been treated with some sulphuric 

 acid. As the composition of nearly every cargo of these guanos 

 varies greatly, the analysis of particular samples is of no general 

 interest. I therefore pass them over as well as the analyses of 

 some other kinds of phosphatic guanos which occasionally find 

 their way into the market. 



9. BONE- ASH. 



Bone-ash of good quality unquestionably is ont of the most 

 valuable materials for making superphosphate of lime. Indeed, 

 it is far too good to be used for agricultural purposes in any 

 other form. Most of the bone-ash of commerce comes from 

 South America, and possesses, like other commercial articles, a 

 variable composition, arising chiefly from the mechanical im- 

 purities, such as sand and earth, with which it is always more or 

 less contaminated. 



The following commercial analyses of different samples of 

 South American bone-ash will give an idea of the extent of this 

 variation : 



