Weight of glasses + pyrites 31.0470 grms. " " glasses 26.0470 " ------- " " pyrites 5.0000 " " " glasses + pyrites, dried 1 hour 30.8965 " " " " " dried 1-1/2 " 30.8957 " " " " " 31.0470 " " " " " dried 30.8957 " ------- " " moisture 0.1513 " 0.1513 × 20 = 3.026 ~3.02%.~ [Illustration: FIG. 3.] Sometimes it may be advisable to dry 10 grams, in which case multiplying the loss by 10 will give the percentage. The dried ore should be transferred to a weighing-tube (fig. 3), and reserved for the subsequent determinations. The weighing-tube with the ore must be marked, and kept in a desiccator. Most ores and inorganic substances can be dried, and their moisture determined by the loss in this way. When, however, the substance contains another somewhat volatile ingredient, it is exposed over sulphuric acid in a desiccator for two days (if _in vacuo_, all the better), and the loss determined. Moisture in dynamite should be determined in this way. When water is simply mechanically mixed with a substance it presents but little difficulty. The combined water is a different matter. Slaked lime, even when perfectly dry, contains much water; and if the water of soda crystals were separated and frozen, it would occupy a volume equal to that of the original crystals. Perfectly dry substances may contain much water, and this combined water is retained by different materials with very unequal vigour. Sodium sulphate and sodium phosphate crystals lose water even when exposed under ordinary conditions to dry air. Soda crystals when heated melt, and at a moderate temperature give off their water with ebullition. The temperature at which all the water is given up varies with each particular salt; the actual determination of the water in each case will require somewhat different treatment. Such determinations, however, are seldom required; and from a practical point of view this combined water causes no trouble.