168 



PRINCIPLES OF CHEMISTRY 



By the aid of the eudiometer we may not only determine the volu- 

 metric composition of water, 32 and the quantitative contents of oxygen 



I'!' 5 - The eudiometer is used for determining the composition of combustible 



gases. A detailed account of gas analysis would be out of place in this work 

 (see Note 30), but, as an example, we will give a short description of the deter- 

 mination of the composition of water by the eudiometer. 



Pure and dry oxygen is first introduced into the eudiometer. When the 

 eudiometer and the gas in it acquire the temperature of the surrounding 

 atmosphere which is recognised by the fact of the meniscus of the mercury 

 not altering its position during a long period of time then the heights at 

 which the mercury stands in the eudiometer and in the bath are observed. 

 The difference (in millimetres) gives the height of the column of mercury in 

 the eudiometer. It must be reduced to the height at which the mercury 

 would stand at and deducted from the atmospheric pressure, in order to 

 find the pressure under which the oxygen is measured (see Chap. I. Note 29). 

 The height of the mercury also shows the volume of the oxygen. The tem- 

 perature of the surrounding atmosphere and the height of the barometric 

 column must also be observed, in order to know the temperature of the oxy- 

 gen and the atmospheric pressure. When the volume of the oxygen has been 

 measured, pure and dry hydrogen is introduced into the eudiometer, and the 

 volume of the gases in the eudiometer again measured. They are then ex- 

 ploded. This is done by a Leyden jar, whose outer coating is connected by 

 a chain with one wire, so that a spark passes when the other wire, fused into 

 the eudiometer, is touched by the terminal of the jar. Or else an electrophorus 

 is used, or, better still, a Ruhmkorff's coil, which has the advantage of work- 

 ing equally well in damp or dry air, whilst a Ley Jen jar or electrical machine 

 does not act in damp weather. Further, it is necessary to close the lower 

 orifice of the eudiometer before the explosion (for this purpose the eudio- 

 meter, which is fixed in a stand, is firmly pressed down from above on to a piece 

 of india-rubber placed at the bottom of the bath), as otherwise the mercury 

 and gas would be thrown from the apparatus by the explosion. It must 

 also be remarked that to ensure complete combustion the proportion between 

 the volumes of oxygen and hydrogen must not exceed twelve volumes of 

 hydrogen to one volume of oxygen, or fifteen volumes of oxygen to one 

 volume of hydrogen, because no explosion will take place if one of the gases 

 be in great excess. It is best to take a mixture of one volume of hydrogen 

 with several volumes of oxygen. The combustion will then be complete. It is 

 FIG. 32. evident that water is formed, and that the volume (or tension) is diminished, 

 Eudiometer. 8O th a ti on opening the end of the eudiometer the mercury will rise in it. 

 But the tension of the aqueous vapour is now added to the tension of the 

 gas remaining after the explosion. This must be taken into account (Chap. I. Note 1). 

 If there remain but little gas, the water which is formed will be sufficient for its satura- 

 tion with aqueous vapour. This may be learnt from the fact that drops of water are 

 visible on the sides of the eudiometer after the mercury has risen in it. If there be none, 

 a certain quantity of water must be introduced into the eudiometer. Then the number 

 of millimetres expressing the pressure of the vapour corresponding with the tempera- 

 ture of the experiment must be subtracted from the atmospheric pressure at which the 

 remaining gas is measured, otherwise the result will be inaccurate. 



This is essentially the method of the determination of the composition of water which 

 was made for the first time by Gay-Lussac 1 and Humboldt with sufficient accuracy. 

 Their determinations led them to the conclusion that water consists of two volumes of 

 hydrogen and one volume of oxygen. Every time they took a greater quantity of oxygen, 

 the gas remaining after the explosion was oxygen. When they took an excess of hydro- 

 gen, the remaining gas was hydrogen ; and when the oxygen and hydrogen were taken in 



