For the present purpose, however, the pan is not necessary, and it can be replaced by the roll of paper and platinum coil as shown in the drawing. [Footnote 11: This balance can be obtained from Nalder Bros. & Co., Westminster.] The method of using is exceedingly simple, as the increase in the length of the spiral is in direct ratio to the increment of weight. The position of the spiral is ascertained by placing the eye in a direct line with the small glass bead C and its image in the mirror, and noting the corresponding division on the scale. The position of the bead can be altered so as to bring it to any desired point on the scale by raising or lowering the upright rod F, which is kept in position by the screw G. The balance stands on a foot provided with levelling screws. It is evident that where _proportional_ weights only are required it is not necessary to know the value of the spiral, but if the balance is to be used to ascertain _actual_ weights, the coefficient of the spiral must first be determined. This is done by noting the increase in length after the addition of a one gram weight. The spirals are made of different thicknesses of wire, which of course give varying degrees {136} of sensibility: the most useful is one which gives with one gram an extension of 100 millimetres: one mm. being therefore equivalent to one mgrm. The following experiment will illustrate the method of using the balance and of calculating the results of a determination of the amount of mineral matter in a paper:— Position of bead on scale 100 mm. Position of bead after attaching platinum wire 151 „ Position of bead with wire and roll of paper attached 200 „ Weight of paper expressed as millimetres 200−151 = 49 „ Position of bead with wire and ash attached 156·5 „ Weight of ash expressed as millimetres 156·5−151 = 5·5 „ Weight of paper Weight of ash Percentage of expressed in expressed in ash in millimetres. millimetres. paper. 49 : 100 :: 5·5 : 11·224 In order to ascertain the percentage of mineral matter _actually added_ to a paper it is necessary to deduct from the amount of ash obtained a certain quantity due to the mineral matter in the fibre of which the paper is composed. This amount varies with each particular fibre, and with the method by which it has been prepared. The following table gives the percentage of ash yielded by the various pulps in a perfectly bleached state. In all cases the fibres were previously treated with a dilute solution of hydrochloric acid, in order to remove any carbonate of lime or other bodies which might have been introduced in the boiling and bleaching processes: the percentages are calculated on the dry substances:—