To illustrate how it works, let us suppose that we are to determine the combination ratio of hydrogen to oxygen in water. The eudiometer is filled with water, it is immersed in a tank containing water and, keeping the top cock closed, equal volumes of hydrogen and oxygen, measured with a graduated cylinder, are introduced through the base. Once the bottom cock has been closed, on bringing the sparking device close to one of the poles of an electrostatic generator, the hydrogen and oxygen combine in part, thus producing water.
To determine the amount of residual gas, a long graduated cylinder closed at the top and full of water is screwed down inside the tank (in our specimen the graduated cylinder is missing). On opening the top cock, the eudiometer fills with water and the residual gas goes into the graduated cylinder; this is then unscrewed and, keeping it closed, it is immersed vertically in a tank full of water. On opening the cock it is possible to measure, at atmospheric pressure, the volume of residual gas: this is found to be one quarter of the volume of the mixture introduced. It is not difficult to establish that it is oxygen and it can thus be concluded that two parts hydrogen combined with one part oxygen by volume.
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