_Grinding Lenses._--This is scarcely within the scope of a book on glass-blowing for laboratory purposes, but it may be said that the lens may be ground by means of a permutating mould of hard lead or type-metal. The rough shaping is done with coarse carborundum or emery, and successive stages are carried on with finer and finer material. The last polishing is by the use of jewellers' rouge on the mould, now lined with a fine textile. _Filing Glass._--If a new file, thoroughly lubricated with a solution of camphor in oil of turpentine, is used, there is but little difficulty in filing the softer glasses. A slow movement of the file, without excessive pressure but without allowing the file to slip, is desirable. After a time the cutting edges of the file teeth will wear down and it will be necessary to replace the file by another. _Grinding Stoppers._--This is, perhaps, the most common form of grinding that the laboratory worker will need to perform, and for that reason, rather full details of the procedure are desirable. A very crude form of ground-in stopper may be made by drawing out the neck and the mass of glass which is intended to form the stopper to approximately corresponding angles, wetting the surfaces with a mixture of the abrasive material and water, and grinding the stopper in by hand. Frequent lifting of the stopper is necessary during grinding, in order to allow fresh supplies of abrasive material to reach the contacts. When an approximate fit is obtained, the coarse abrasive should be washed off, care being taken that the washing is complete, and a finer abrasive substituted. After a while, this is replaced in its turn by a still finer grinding material. Such a method of grinding may give a satisfactory stoppering if the angles of the plug and socket correspond very closely before grinding is commenced; but if there is a wide difference in the original angles, then no amount of grinding by this method will produce a good result. The reason for this is that the plug will become so worn in the preliminary grinding as to assume the form of a highly truncated cone; the socket will assume a reverse form, and the end result will be a loose-fitting plug and socket.