428 On the Chemical Composition of Phosphatic Minerals 
alumina, or probably both. Navassa phosphate, it will be seen, 
contains rather much oxide of iron and still more alumina, two 
constituents which lower the commercial value which it would 
otherwise possess, inasmuch as superphosphate, made from this- 
material, becomes poorer in soluble phosphate of lime, after it 
has been kept some time, than it was when first made. This 
remark applies to all raw phosphatic minerals containing much 
oxide of iron or alumina. The acid employed in dissolving 
such crude phosphatic minerals at first attacks mainly the 
phosphate of lime, and changes it into soluble phosphate and 
sulphate of lime ; but, on keeping, the acid soluble phosphate 
reacts upon oxide of iron and alumina, and to some extent parts 
with its acid, which, combining with the latter to form insoluble 
phosphate of iron and alumina, causes the precipitation of a cor- 
responding quantity of insoluble phosphate of lime, or in other 
words, iron and alumina lead to the reduction of soluble phos- 
phate of lime in superphosphate. 
The following analyses express the composition of some 
samples imported into England during the last two years : — 
Composition of Navassa Phosphates. 
No. 1. 
No. 2. 
No. 3. 
No. 4. 
No. 5. 
No. 6. 
Moisture, water of combina-'l 
lion, and loss on heating . . / 
Magnesia, carbonic acid, oxide j 
of iron, alumina, &c / 
Insoluble siliceous matter .. 
* Equal to tribasic phos-l 
12-08 
30-21 
35-32 
19-G5 
2-74 
10-90 
31-08 
36-54 
17-78 
3-70 
13-99 
30-04 
35-99 
17-04 
2-94 
12-55 
31-90 
36-09 
16-91 
2-55 

9-35 
31-85 
37-91 
17-99 
2-90 
10-53 
29-60 
31-7-2 
25-45 
2-70 
100-00 100-00 
100-00 
100-00 
100-00 
100-00 
65-94 
67-85 
65-58 
69-64 
69-53 
64*62 
13. St. ]Martin's Phosphates. 
Rock-phosphates are also found on St. Martin, a small island 
belonging to the group of Windward Islands, situated 18-5° north 
latitude, and 63-4^ west longitude, in the West Indian Sea. 
The character of the phosphate deposits on St. Martin varies 
a great deal, and care and attention have to be exercised in mining 
and separating the more valuable and richer deposits from the 
inferior minerals which are intermixed with the coral rock on 
Avhich the phosphate is deposited. 
The variable character of these phosphatic minerals will be 
recognised by an inspection of the following analyses which I 
made of a number of samples from St. Martin : — 
