text:  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