"In a former paper, in which we dwelt much on this subject, we mentioned that we had the report of a very scientific friend, who had spent nearly a life of leisure and competence in experiments on pictures, that the paint of the old masters _fused_, not only where white lead had been used, but in every part; and we ourselves saw him try the experiment upon the background of an old picture, by means of the blow-pipe, and the result was a fused substance -- a glass."