_Methods of Calibration._--In the case of apparatus for volumetric work, this is usually carried out by weighing, although some of the smaller subdivisions are often made by measurement. When the subdivisions are made in this way it is of importance to see that the walls of the tube or vessel to be calibrated are parallel. Great errors arise in some of the commercial apparatus from neglect of this precaution. A convenient method of testing for parallelism, in the case of a wide tube, is to close one end and to weigh in successive quantities of mercury. An observation of the length occupied by each successive quantity will indicate any change in the bore. In the case of capillary tubes, it is convenient to introduce an unweighed quantity of mercury, measure its length accurately, and then to move it along the tube in stages, either by tilting the tube or by the application of air pressure. A measurement of the length at each stage will indicate whether the bore is approximately parallel or not. Neither of these methods is to be relied on without a careful examination of the tube, as it may happen that there are local irregularities in the bore which compensate for each other, and do not, therefore, affect the volume of a given length. Obviously, the smaller the quantity of mercury with which the test is carried out and the greater the number of observations made, the less risk will there be of such an error. A liquid, such as water or alcohol, which wets the glass is not suitable for such a test, unless special precautions are taken. When, however, a pipette or burette has to be calibrated to deliver a certain volume of water, the final calibration must be made with this liquid. Thus, the burette would first be calibrated by weighing in definite quantities of mercury of say 13.54 grammes (1 cc at 15°C.), each of the 1 cc divisions should be marked by some temporary marking. The burette is now filled with a solution of potassium bichromate and sulphuric acid and allowed to soak for some time; the bichromate is washed out and distilled water is put in. Successive quantities of water are run out of the jet, a fixed time being allowed for draining, and the weights of the quantities delivered are noted. This procedure will give the necessary data for altering the marking so that it may correspond to 1 cc _delivered_. Each 1 cc division is now divided into tenths by the method described below. A final verification of the markings should be made when the subdivision is completed. _Subdivision of Graduations._--Mark out the spaces to be subdivided on a sheet of paper. Take a reliable ruler on which any convenient length is divided into the desired number and place it across the lines at such an angle that the limits noted on the rule exactly bridge the gap. Now draw parallel lines through the markings.