Metallic zinc is readily soluble in dilute hydrochloric or sulphuric acid, hydrogen being at the same time evolved.[74] The volume of the hydrogen evolved is obviously a measure of the amount of zinc present in the metallic state. The speed with which the reaction goes on (even in the cold) and the insolubility of hydrogen renders this method of assay a convenient one. It is especially applicable to the determination of the proportion of zinc in zinc dust. The apparatus described in the chapter on gasometric method is used. The method of working is as follows: Fill the two burettes with cold water to a little above the zero mark, place in the bottle about 0.25 gram of the substance to be determined, and in the inner phial or test tube 5 c.c. of dilute sulphuric acid; cork the apparatus tightly and allow to stand for a few minutes; then bring the water to the same level in the two burettes by running out through the clip at the bottom. Read off the level of the liquid in the graduated burette. Turn the bottle over sufficiently to spill the acid over the zinc, and then run water out of the apparatus so as to keep the liquid in the two burettes at the same level, taking care not to run it out more quickly than the hydrogen is being generated. When the volume of gas ceases to increase, read off the level of the liquid, deduct the reading which was started with; the difference gives the volume of hydrogen evolved. At the same time read off the volume of air in the "volume corrector," which must be fixed alongside the gas burettes. Make the correction. For example: A piece of zinc weighing 0.2835 gram was found to give 99.9 c.c. of gas at a time when the corrector read 104 c.c.[75] Then the corrected volume is 104 : 100 :: 99.9 : _x_. _x_ = 96.0 c.c. 100 c.c. of hydrogen at 0° C. and 760 mm. is equivalent to 0.2912 gram of zinc; therefore the quantity of zinc found is 100 : 96 :: 0.2912 : _x_. _x_ = 0.2795 gram of zinc. This being contained in 0.2835 gram of metal is equivalent to 98.5 per cent. As an example of a determination in which reducing the volume of liberated hydrogen to 0° C. and 760 mm. is avoided, the following may be taken:-- 0.2315 gram of pure zinc gave 82.1 c.c. of gas; and the volume of air in the corrector was 103.6 c.c. 0.2835 gram of the assay gave 99.9 c.c. of gas; and the volume of air in the corrector was 104.0 c.c.; 104 : 103.6 :: 99.9 : _x_. _x_ = 99.5 c.c. This is the volume of gas got in the assay if measured under the same conditions as the standard, 82.1 : 99.5 :: 0.2315 : _x_. _x_ = 0.2806. Then 0.2835 : 0.2806 :: 100: _x_. _x_ = 98.9 per cent.