THE MANUFACTURE OF ACID PHOSPHATE. 11 
The advantages of having small quantities (and in some cases 
large quantities) of carbonate of lime present in phosphate rock are 
threefold: First, the heat evolved in the reaction between carbonates 
and sulphuric acid is sufficient to warm the pasty mass of acid and 
phosphate rock and thus promote chemical action between these 
more slowing reacting substances; second, the escape of carbon 
dioxide from the mass renders the acid phosphate porous and more 
readily dried; and third, the gypsum formed prevents the formation 
of the gelatinous iron and aluminum compounds and thus helps 
render the product dry and in good condition for distributing or 
mixing with other fertilizing ingredients. 
REVERSION OF SUPERPHOSPHATE. 
The reversion of superphosphate, as the term implies, originally 
meant the return of the phosphoric acid to a condition insoluble (or 
nearly so) in water. In reality the expression "reverted" phosphoric 
acid is now wrongly used in a much broader sense and includes all of 
the phosphoric acid of superphosphate which is soluble in certain 
citrate solutions. In this paper, however, reverted phosphoric acid 
is used in the strict sense of the word. 
When, a superphosphate is allowed to stand and take up water 
from moist air, as it sometimes does while in storage; or is, diluted by 
the soil water when it is applied to the soil; or is added to an excess 
of water, as is done in the laboratory before commencing analytical 
operations, then, in any one and in all of these cases, less soluble 
compounds of phosphoric acid are formed. If compounds of iron 
and aluminum are present the formation of phosphates insoluble in 
water is much more marked. This general process is known as 
reversion, and the superphosphate is said to have reverted, and the 
product is called reverted jphospJiate. The theory of this reversion is 
now clearly understood, owing to the investigations in this country 
of Cameron and Bell 1 and Seidell, 2 and of Bassett 3 in England, who 
have shown that certain concentrations of phosphoric acid or of other 
acids must exist in the water in contact with a calcium or iron phos- 
phate for the solid definite ' ' acid" compounds to be stable. Dilution 
of the acid liquor causes the solids to decompose into more basic and 
less soluble compounds. While the theory of these phenomena has 
been made clear only recently, the main facts have long been known, 
and as is so commonly the case, certain popular misconceptions have 
held sway long enough to become regarded as facts even by many well- 
trained chemists. Thus, it is popularly held that monocalcium phos- 
phate is soluble in water, but dicalcium phosphate is not; dicalcium 
i Bui. 41, Bureau of Soils, U. S. Dept. Agr. (1907). 
2 Jour. Am. Chem. Soc, 27, 1503 (1005). 
3 Chem. News, 95, 21 (1907); Zeit. anorg. Chem., 53, 34 (1907). 
