240 ELEMENTARY GENERAL SCIENCE CHAP. 



equal volumes, usually cubic centimetres, by divisions marked 

 on the glass. Through opposite sides of the tube at the closed 

 end pieces of platinum wire are passed and fused into the 

 glass, being so arranged that they do not quite touch one 

 another. Outside the tube the platinum wires are bent into 

 loops to which wires from an electric coil may be attached. 



Composition of Water by Volume. To use the eudiometer 

 it is first completely filled with mercury and inverted over more 

 mercury contained in a trough. A suitable quantity of pure 

 dry oxygen is then bubbled into the tube and the volume (after 

 the necessary corrections for temperature and pressure) is 

 recorded. Pure dry hydrogen is next bubbled into the tube, 

 using a considerable excess of one or other of the gases. The 

 volume is again recorded (with necessary corrections as 

 before) and then, keeping the eudiometer firmly pressed upon a 

 sheet of india-rubber, or felt, at the bottom of the trough, the 

 gases are made to combine by causing an electric spark to pass 

 between the platinum wires inside the tube. As soon as the 

 spark passes, the two gases combine, with a flash of light. The 

 eudiometer is slightly raised from the india-rubber (but of course 

 not above the mercury in the trough), and it is seen that the 

 volume of the gas in the eudiometer is less than before the 

 explosion, and that there is a film of moisture upon the interior 

 of the tube; The corrected volume is again recorded and the 

 nature of the gas ascertained. We then find the volumes of the 

 gases which have combined in the manner indicated below : 



Corrected volume of oxygen ... 12 c.c. 



Corrected volume of mixed gases ... 50 c.c. 



Therefore corrected volume of hydrogen = 38 c.c. 



Corrected volume after explosion = 14 c.c. 



Gas left ascertained to be Hydrogen. 



Hence the 12 c.c. of oxygen united with (38-14), i.e., with 

 24 c.c., of hydrogen, and we find this result always obtains, 

 namely, 2 volumes of hydrogen combine with 1 volume of 

 oxygen to form water. 



If instead of doing the experiment in the above manner the 

 eudiometer tube be kept heated above 100 C. during the experi- 

 ment (by surrounding it with a larger tube through which the 

 vapour of some boiling liquid was passed), it would be further 

 found that the steam produced from these 3 volumes of oxygen and 

 hydrogen would only occupy 2 volumes. 





