EXPERIMENTS IN THE CHEMISTRY. 73 



pail, and then slipping a plate underneath each one, as shown 

 in the second figure on p. 22, leaving enough water on the 

 plate to cover the edge of the bottle ; it may then be lifted 

 out and placed in the cistern. In the same way the bottles, 

 when filled with gas, may be remc ved and kept for use. Gas 

 may be passed from one bottle to another by inverting one 

 over the other beneath the water in the pneumatic cistern, 

 or in a large pail, when the gas will bubble up from the lower 

 one into the upper one. 



Apply the heat to the glass retort very carefully at first, 

 holding the lamp in the hand and moving it around so that 

 the flame may strike all the lower part of the flask, and thus 

 expand it uniformly. Be careful also that there is no draft of 

 cold air to strike against the heated retort. With the cop- 

 per retort no care of this kind is necessary. When the gas 

 ceases to come off, remove the stopper or lift the end of the 

 tube out of the water; otherwise, as the retort cools and a 

 vacuum is formed, the water in the cistern will set back into 

 the flask, and, if of glass, will break it. An ounce of the salt 

 will make over six quarts of oxygen gas. When the retort is 

 oartly cooled, pour in some warm water to dissolve the residu- 

 um, which may then be poured out and the retort drained and 

 set away for future use. In order to test the purity of the 

 materials, and thus avoid any danger of an explosion, place a 

 little of the mixture for making oxygen in an iron spoon and 

 aeat it over the spirit-lamp. If the gas passes off quietly, no 

 langer need be apprehended. 



22. i. The experiment with the candle can be performed 

 nost strikingly by filling a common fruit-jar with nitrogen 

 v see page 32) and another with oxygen. The covers will pre- 

 5erve the gases until wanted for use. The covers may then 

 be laid loosely on top cf the jars, and the lighted candle 

 passed quickly from one jar to the other. It will be ex- 

 anguished in one and relighted in the other. With care, it 

 may be passed and repassed a dozen times. This strikingly 

 illustrates the difference between oxygen and nitrogen. Test 

 the carbonic acid, in this as in all similar experiments, with 

 the blue litmus and the green cabbage, or a slip of blotting- 

 paper wet with the litmus solution. A few drops of the solu- 



