282 CHEMICAL MANUEES. 



presence alone diminishes the value of the guano of the superficial 

 bed, besides it dissolves salts of potash, ammonia, lime and magnesia 

 in so far as they are soluble and causes them to pass into the middle 

 bed. To this downward transition there is a corresponding ascend- 

 ing transition from the lower beds. In fact the guano of the 

 lower bed in contact with the soil is in a state of incessant 

 decomposition, and gives off ammonia and carbonate of ammonia, 

 which being very volatile ascend towards the top. The two opposite 

 currents meet in the middle zone. The dissolved earthy salts fix. 

 the ammonia, which is thus converted into stable compounds. It is, 

 therefore, easy to understand that the middle bed ought to be the 

 richest in nitrogen, especially in ammoniacal salts. If this transfer 

 of matter in itself sufficiently explains the great differences in com- 

 position revealed by analysis, it is not the sole cause. Peruvian 

 guano does not wholly consist of the excrement of sea-fowL 

 Remains of seals and other marine animals are found in the lower 

 beds in a state of curious preservation. It is, therefore, right tO' 

 admit that these islands at a previous epoch when they w^ere lower,, 

 served as a refuge to a great number of these animals, and that 

 after they had become inaccessible owing to their volcanic upheaval 

 they were the home of sea-fowl. This hypothesis enables us tO' 

 understand whv the browm euano of the lower bed is of less value ; 

 for the flesh and the decomposed bones of w^hich it consists are less 

 rich in nitrogen than the birds' excrement. The composition of 

 guano, therefore, varies according to whether it comes from the 

 upper, the middle, or the lower bed of the deposit. The concretions 

 (nodules) of the upper bed have a composition identical with that of 

 the powder in which they are embedded. Their colour and their 

 structm-e enable one at first sight to recognize the guano from the 

 surface bed. The nuts of guano from the middle bed are dis- 

 tinguished from the surrounding mass by their colour, which is. 

 paler, sometimes almost w^hite, and they contain a crystalline nucleus. 

 or have a crystalline structure here and there. They consist chiefly 

 of ammoniacal salts, oxalate and carbonate, and consequently con- 

 tain more nitrogen than the guano in w^hich they are embedded.. 

 The difference is as much as about I'D per cent, and much more 

 in the case of very pure pieces. The nodules, the analyses of 

 which are given here, according to Karmrodt and Phipson, come 

 undoubtedly from the middle bed. They contain : — 



