216 



Composition and Value of Guano. 



could be collected and sold, as in the other case, ground at 2/. 

 a ton, which at first sight appears a considerably cheaper rate 

 than the Suffolk kind. 



They contain on an average only 42J per cent, of phosphate 

 of lime : from this source a ton will consequently cost 41. 14s. lj^.. 

 at the rate of 4s. 2\d. for 100 lbs., or as nearly as can be \d. a lb.* 



Allowing for all imperfections in these data, we shall perhaps 

 not be far wrong if we fix the cost of phosphate of lime derived 

 from mineral sources at \d. a lb. 



But the argument is by no means complete at this point; it is 

 not to be supposed that the substance in question, as found in the 

 coprolite, or green-sand fossils, is equal insolubility, and therefore 

 to be ranked in value with that contained in guano. From the 

 fact that, in the latter case, it is derived from the pulpy excrements 

 of birds (deposited centuries ago, indeed, but preserved com- 

 paratively unchanged) we should be disposed to give it a high 

 place in the scale of solubility, whilst a glance at the mineral 

 substance would convey the very opposite impression. 



My own belief is, that of the two varieties of mineral phos- 

 phates, simply ground, the green-sand variety would prove the most 

 energetic upon vegetation ; but I am bound to say that it would 

 be an error, in the present state of our knowledge, to place either 

 of them in competition in this respect with the earthy phos- 

 phate of guano. It will occur to the reader that there are means 

 of rendering phosphate of lime soluble, and thus enabling it to 

 equal and far even to exceed the same substance as supplied by 

 guano. In superphosphate of lime we are familiar with phosphate 

 of lime so prepared and rendered soluble, and it might be in- 

 teresting to inquire at what cost we can purchase it in this soluble 

 and unexceptionable form. 



From calculations, which need not be here introduced, I be- 

 lieve that phosphate of lime, as supplied by " superphosphate," 

 is bought at a price varying from \\d. to 2\d. per lb., according 

 to the mode in which the so-called superphosphate is manufac- 

 tured. This substance, however, has, from its solubility, a 

 specific action on the turnip crop which would hardly seem to be 

 equalled by any other form of phosphate of lime, and it is 

 therefore of very little use to place it in comparison with that of 

 guano. Of one circumstance, however, we may rest assured, that 

 phosphate of lime in any form cannot be worth more than about 



* Since the above was written, Mr. Paine has informed me that he has this year 

 dug a quantity of these phosphoric nodules at a much less expense. If at any parti- 

 cular spot a bed of them should be found within a foot or two of the surface, and 

 running parallel to it, I believe that it would well repay the expense of working, but 

 hitherto the nodules have been found in strata which, although tolerably horizontal 

 themselves, are only accessible at the outcrops on the edges of undulating ground. 



