APPENDIX 325 



of the delivery-tube and its stopper, upon one pan of the balance, 

 and exactly counterpoise it with sand, poured into a beaker upon 

 the other pan. This counterpoise must not be disturbed until the 

 experiment has been completed. Connect the apparatus up with a 

 delivery-tube and support it so that the heating-tube is in a slanting 

 position, while the end of the delivery-tube dips under the mouth of 

 a large bottle filled with water, and set inverted upon the shelf of 

 the pneumatic trough. Gradually heat the mixture in the tube, 

 holding the burner in your hand, and keeping the flame brushing 

 along the tube. Once heating is commenced, it must not be inter- 

 rupted until no more gas is seen to leave the delivery-tube : remove 

 the latter, with its stopper, from the U-tube; and then take away the 

 flame and allow the apparatus to grow cold. (Removing the flame 

 before the delivery-tube was disconnected might cause water to rise 

 back into the apparatus, owing to the contraction of the cooling 

 gas.) When the apparatus is cold, place the same parts upon the 

 balance pan, as before: but a certain amount of weights must be 

 added to this pan, to bring it into equilibrium with the original 

 counterpoise. This weight should be noted, as it represents the mass 

 of oxygen which has been transferred from the tube to the bottle. 

 We must now proceed to measure the volume occupied by this 

 mass. Note the temperature of the air near the bottle. Raise the 

 bottle slightly from its shelf, slip a piece of smooth paper under its 

 mouth, and invert dexterously, without spilling any of the water 

 from the bottle. No effort need be made to retain the oxygen, as 

 its volume is indicated by the contents of the bottle above the 

 water level. Fill the bottle brimful from a measuring-glass, being 

 careful to ascertain the number of cubic centimeters of water re- 

 quired to complete the filling, as they represent the volume of oxy- 

 gen at the temperature of the room, t. To reduce this volume to 



O^Q 



0, multiply it by the fraction - 



273 +2 



Knowing the volume and the mass of the oxygen produced in 

 this experiment, calculate the weight of one liter of oxygen gas. 



RELATIONS OF COMBINING WEIGHTS 



EXP. 15. Weigh a clean watch-glass, place about half a gram 

 of granulated zinc upon it, and weigh accurately. The difference 



