Compute the percentage of errcr, as in the case of O. If the purity of the HCl be known, i.e. the weight of HCl gas in one cubic centimeter of the liquid, a proportion can be made between HCl and H, provided no free HCl is left in the flask. State any liabilities to error in this experiment. PROBLEMS. (1) A gas occupies 2000cc.when the barometer stands 750mm. What volume will it fill at 760mm? (2) At 750mm my volume of O is 4 1/2 liters. What will it be at 730mm? (3) At 825mm? (4) At 200mm? (5) Compute the volume of a gas at 70 degrees, which at 30 degrees is 150cc. (6) At 0 degrees I have 3000cc.of O. What volume will it occupy at 100 degrees? (7) I fill a flask holding 2 litres with H. The thermometer indicates 26 degrees, the barometer 762mm. What is the volume of the gas at 0 degrees and 760mm? If the volumes of gases vary as above, it is evident that their vapor densities must vary inversely. A liter of H at 0 degrees weighs 0.0896. What will a liter of H weigh at 273 degrees? At 273 degrees the one liter has be- come two liters, one of which weighs 0.0448 (= 0.0896 / 2). The vapor density of a gas is inversely proportional to the temperature. Also, the vapor density is directly proportional to the pressure, since a liter of any gas under a pressure of one atmosphere is reduced to half a liter under two atmospheres. PROBLEMS. (1) Find the weight of a liter of O at 0 degrees; then compute the weight of a liter at 27 degrees. (2) Find the weight of 500cc.of N2O at 60 degrees. (3) Of 200 cc. of CO at -5 degrees. (4) A given volume of O weighs 0.25g at a pressure of 750mm; find the weight of a like volume of O at 758mm. APPENDIX. INDIVIDUAL APPARATUS. Each pupil should be provided with the apparatus given below, but in cases where great economy must be exercised different pupils may, by working at different times, use the same set. The author has selected apparatus specially adapted, as to exact dimensions, quality, and cheap- ness, for performing in the best way the experiments herein described, and sets or separate pieces of this, together with other apparatus and chemicals, can be had of the L.E. Knott Apparatus Co., 14 Ashburton Place, Boston, to which firm teachers are referred for catalogs.