With a finer class of papers common materials are as readily employed, through the assistance of some colouring matter, which tends to conceal the imperfection. Indeed, it would be difficult to name an instance of apparent deception more forcible than that which is accomplished by the use of ultramarine. Until very recently the fine bluish tinge given to many writing papers was derived from the admixture of that expensive mineral blue, the oxide of cobalt, generally termed _smalts_, and which has still the advantage over the ultramarine of imparting a colour which will endure for a much longer period. One pound of ultramarine, however, going further than four of smalts at the same price, the former necessarily meets with more extended application, and where the using is rightly understood, and the materials employed instead of being fine rags, comparative rubbish, excessively bleached; its application proves remarkably serviceable in concealing for a time all other irregularities, and even far surpassing in appearance the best papers of the kind. At first the introduction of ultramarine led to some difficulty in sizing the paper, for so long as smalts continued to be used, any amount of alum might be employed, and it was actually added to the size to preserve it from putrefaction. But since artificial ultramarine is bleached by alum, it became of course necessary to add this salt to the size in very small proportions, and as a natural consequence, the gelatine was no longer protected from the action of the air, which led to incipient decomposition, and in such cases the putrefaction once commenced, proceeded even after the size was dried on the paper, and gave to it a most offensive smell, which rendered the paper unsaleable. This difficulty, however, has now been overcome, and providing the size be quite free from taint when applied to the paper, and quickly dried, putrefaction will not subsequently occur; but if decay has once commenced, it cannot be arrested by drying only.