COMMON THINGS. WATER. 



Fig. 2. 



or the other gas in its separate and pure state. Whether it bo 

 pure and unmixed oxygen, or pure and unmixed hydrogen, will 

 depend on the proportions in which the gases were originally 

 mixed in the vessel before the explosion. 



41. There are many forms of apparatus by means of which this 

 important experiment may be performed. One of them is repre- 

 sented in fig. 2. 



A cylindrical vessel, D E, wider at the top than below, is filled 

 with mercury. A graduated tube, B c, of thick and strong glass, 



about an inch in diameter, 

 closed at one end, B, and 

 open at the other, c, being 

 filled with mercury and 

 stopped by the hand at the 

 open end, is inserted and 

 plunged in the mercury in 

 the cistern, D E. The mer- 

 cury will not fall out of B c, 

 because the atmospheric 

 pressure acting on the ex- 

 ternal surface of the mer- 

 cury in D E will support it. 

 The gases, oxygen and 

 hydrogen, may now be 

 introduced into B c, by dis- 

 charging them in the mer- 

 cury under the open mouth 

 of the tube, B c. They will 

 rise in bubbles through the 

 mercury, and will displace 

 a portion of that liquid in 

 the top of the tube, B c. In this manner any desired proportions 

 of the gases may be introduced into the tube, B c, limited only by 

 the capacity of the tube. 



Near the top of the tube, B c, two small holes on opposite sides 

 are bored, through which two pieces of platinum wire are inserted, 

 terminating inside and outside in knobs. The inside knobs are 

 close to each other, without being actually in contact. When the 

 knob of a charged electric jar is presented to B, while A is con- 

 nected by a metallic chain with the outside coating of the jar, the 

 electric discharge will pass between the two inner knobs, and will 

 inflame the hydrogen contained in the tube, B c. 



It is in experimental researches of this kind more convenient to 

 express the quantities of the gases by their measures as indicated 

 by the graduation of the tube, B c. It will render this explanation, 

 108 



