The astringent principle is a peculiar kind of tannic acid, called by chemists quercitannic, which, yielding more stable compounds with gelatine than other forms, gives oak bark its high value to the tanner.
By using urea, guanidine, thiourea and related compounds instead of amidines, one obtains the uracils.
For the preparation of yttrium compounds the best raw material is gadolinite, which, according to Kiinig, consists of 22.61% of silica, 34.64.
Molybdenum combines with the halogen elements in varying proportions, forming with chlorine a di-, tri-, tetraand penta-chloride, and similar compounds with bromine and iodine.
