300 Domestic Science 



gases is then read off as before. A further quantity of 

 mercury is then poured into the open limb so as to fill 

 it within 6" of the top and this limb is closed with a tight- 

 fitting rubber stopper. An electric spark is then caused 

 to pass between the ends of the wires fused into the upper 

 end of the closed limb. This is effected by connecting 

 the outer ends of the wires to the wires from a small 

 induction coil, and momentarily switching on the 

 current. The passage of the spark causes explosive 

 combination to occur, and on removing the stopper 

 from the open limb, a contraction in volume of the 

 gases in the other limb is seen to have taken place. 

 On reading off the enclosed volume after levelling the 

 surfaces of the mercury in the two limbs, the decrease 

 of volume is found to be three times that of the oxygen 

 used. The residual gas, on being tested, may be shown 

 to consist entirely of hydrogen. Two volumes of hydro- 

 gen have therefore combined with one volume of 

 oxygen to form steam, and, since the volume of water 

 formed from this steam has an almost inappreciable 

 volume as compared with that of the steam (see 

 Chapter x, par. 114), the decrease in volume of the 

 gases in the closed limb of the eudiometer amounts 

 to thrice the volume of the oxygen. 



If we desire to know the relation between the volumes 

 of the gases and that of the steam produced by their 

 combination, the closed limb must be surrounded by 

 a jacket, as in Fig. 101. Through this jacket is passed, 

 during the experiment, the vapour of some liquid 

 which boils at a temperature considerably higher than 

 the boiling-point of water. Amjd alcohol is sometimes 

 used, its boiling-point being 132 C. A condenser is 

 attached to the jacket in order that the alcohol may 

 be recovered for further use and that its fumes, which 



