636 
PHOSPHATES USED IN AGRICULTURE. 
salts of ammonia are washed out, and the mineral constituents 
increase in proportion; the guano becomes less valuable as 
a manure, by loss of its ammoniacal compounds, but consti¬ 
tutes a plentiful source of phosphate of lime. Such are the 
phosphates known as “ West India phosphate,” “ Bolivian 
guano,” &c. These contain from 40 to 60 per cent, (and 
sometimes more) of ordinary phosphate of lime, whilst their 
per-centage of nitrogen (ammonia) dwindles down to 2, 1, or 
even 0 5 per cent., as the phosphate increases. Here, then, 
is an abundant source of phosphate of lime. 
But several West India islands furnish a species of hard 
rock, of very peculiar aspect, consisting chiefly of phosphate 
of lime. Many persons consider that this rock has been 
derived from guano, supposing it to be the result of exposure 
to the atmosphere for thousands of years; others imagine it 
to be guano modified by volcanic action. I have examined 
this mineral phosphate,* and find that it contains not only 
phosphate of lime, but also a considerable proportion of phos¬ 
phate of alumina—a substance not met with in guano; it is, 
in fact, a compound of phosphate of lime and phosphate of 
alumina, containing about 17 per cent, of the latter, and 
65 per cent, of the former. As this rock is principally de¬ 
rived from the little island of Sombrero, I called it Sombrerite. 
This is another tolerably abundant source of phosphate of lime, 
much used in the manufacture of superphosphate manure. 
Another hard phosphatic rock, of a similar description, is 
found upon M onk Island, in the Gulf of Venezuela. Although 
I have received for analysis in my laboratory many hundred 
specimens of the different phosphates mentioned in this 
paper, I have never yet met with this one from Monk Island; 
but I have reason to believe it is a substance similar in ail 
respects to Sombrerite. Whether it be so or not cannot be 
determined by the few incomplete analyses that appear to 
have been made of it hitherto. However, it constitutes a 
cheap source of phosphate to manufacturers of superphos¬ 
phate manure; and it appears to contain 78 to 80 per cent, 
of phosphate of lime. 
Another, and most abundant source of phosphate of lime 
is, I am happy to say, an indigenous one, and one which is 
very extensively utilized in the manufacture of superphos¬ 
phate. I allude to the Cambridge and Suffolk coprolites . 
These are hard nodules, somewhat cylindrical, and having 
rounded edges. The Cambridge coprolites are found in the 
Upper Green Sand, wdiere they form extensive deposits, and 
are so intimately mixed, on their surface, with the green sand 
* • Journal of the Chemical Society/ 1862. 
