6 BULLETIN 350, U. S. DEPARTMENT OF AGRICULTURE. 
vents used are such as will readily dissolve the oil. Among those em- 
ployed are gasoline, ether, benzene, petroleum ether, chloroform, car- 
bon bisulphid, and carbon tetrachlorid. Apparatus of the continuous- 
extraction type is usually employed. This admits of the recovery of 
the solvent and minimizes the loss of the solvent. It has been reported 
that recent improvements in method and apparatus have made this 
process cheaper than the pressure method. The solvent can be entirely 
removed from both the oil and the residue. It is also held that by the 
employment of this process various kinds of material can be treated 
in the same apparatus. The expense of operating is also said to be 
less than the pressure method. 
The color of oils extracted by solvents is often very dark and it is 
necessary to remove it, but this can usually be accomplished with no 
great difficulty. 
The solvents perhaps most frequently employed, because of their 
comparatively low price, are benzene, petroleum ether, gasoline, and 
carbon tetrachlorid. Of these, carbon tetrachlorid has within recent 
years become very popular, not only because of its cheapness but 
because of its noninflammability and less volatile nature, thus reduc- 
ing the danger from fire and minimizing the loss from evaporation. 
According to a recent report x benzine is used for the extraction of 
soy-bean oil in a new mill of the South Manchurian Railway Co. at 
Dairen, this mill having a capacity of 80 tons daily. 
Hydraulic pressure is perhaps more universally used than any 
other method and especially in the case of seeds- having a high per- 
centage of oil. This method is not practicable, however, with seeds 
of low oil content, because the resulting press cake retains a consider- 
ble percentage of the oil. and this would materially affect the total 
yield. Seeds which contain as high as 30 to 50 per cent of oil are 
best manipulated by extraction under pressure from hot meal or with 
heated plates, a maximum percentage of oil being thereby obtained. 
As a general rule, the quality of an oil obtained by hydraulic pres- 
sure excels that obtained by volatile solvents, cold pressure produc- 
ing even better quality than hot pressure. Oils obtained by the pres- 
sure method invariably contain less impurities than those extracted 
by solvents, and therefore require less refining. 
Suitable presses for extracting fixed oils are on the market, all 
embodying the same general principle. Continuous extractors, pref- 
erably of copper, are also obtainable from American manufacturers, 
especially manufacturers of pharmaceutical machinery. 
EXTRACTION OF OIL FEOM THE PITS BY SOLVENTS. 
A quantity of dry pits was finely ground in a drug mill and intro- 
duced into a continuous-extraction apparatus' fashioned after the 
1 Parlett, H. G. New bean-oil extracting mill at Dairen on the benzine system. In 
Bd. Trade Jour. [London], r. 86, no. 923, p. 385, 1914. 
