2 BULLETIN 144, U. S. DEPARTMENT OE AGRICULTURE. 
the whole process, including the preparation of the raw materials 
used, the methods of manufacture with the chemical reactions 
involved, the equipment of the modern factory, the disposal of 
obnoxious gases, the methods of drying, storing, and disintegrating 
the superphosphate, and the cost and disposal of the finished product. 
RAW MATERIALS. 
The raw materials used in the manufacture of acid phosphate are 
bone, guano, apatite, phosphate rock, and sulphuric acid. 
Before the discovery of the vast deposits of phosphorites or natural 
phosphate. rock in this country bone was one of the farmer's chief 
sources of phosphoric acid. The bones were either steamed, charred, 
or burned and applied directly to the field, or after grinding were 
made into acid phosphate by treating with an approximately equal 
weight of sulphuric acid. 
Ground bone, however, has considerable agricultural value with- 
out being acidulated, and besides, the cost of the phosphoric acid 
contained therein is so much greater than that contained in phosphate 
rock that it is obviously not economical to use the former material 
in the manufacture of acid phosphate. The amount of this substance 
now derived from bone is therefore relatively small. 
Guano is another substance which has been extensively used in 
the manufacture of acid phosphate. This material consists essen- 
tially of the excrements of birds and sometimes of bats, and at one 
time was found in large quantities. There are two types of guano 
deposits: (1) The unleached deposits which are usually found in 
caves or other sheltered places where the droppings have been pro- 
tected from the leaching effect of percolating water. Such a deposit 
not only contains phosphoric acid in a readily available form, but 
also carries considerable quantities of nitrogen, the fertilizer constit- 
uent commanding the highest price. (2) That which has been 
leached of its more soluble constituents by exposure to the weather. 
It contains practically no nitrogen and its phosphoric-acid content, 
though usually high, is relatively insoluble. Deposits of guano have 
been eagerly sought, and accessible and valuable ones are now rather 
scarce. Only those containing high percentages of nitrogen, or situ- 
ated in regions having excellent transportation facilities, are able to 
.compete with other and cheaper sources of phosphate. 
At one time apatite was largely used in the manufacture of acid 
phosphate. This mineral is very widely distributed, and occurs in 
rocks of various kinds and ages. It is most common, however, in 
rocks of the metamorphic crystalline variety, such as limestone, 
gneiss, mica, schist, beds of iron ore, etc. There are two main vari- 
eties of apatite, namely, chlor-apatite (CaClCa^PgO^) and fluor- 
