In using the hydrometer, a portion of the liquid whose specific gravity is to be measured should be placed in a glass cylinder of such a size that the hydrometer when placed in the cylinder will be free to move up and down without coming in contact with the walls of the vessel.

The liquid should be well stirred. For specially accurate work, the temperature of the liquid should be observed by means of a thermometer placed directly in the liquid; when the temperature has become fairly constant, the readings on the hydrometer may be taken.

The eye should be placed on a level with the surface of the liquid and the line where this surface appears to cut the stem of the hydrometer should be taken as the reading of the hydrometer.

In case the liquid is not sufficiently transparent to allow the scale of the hydrometer to be read through the liquid, the reading cannot be made as indicated above. It is then necessary to read as accurately as possible above the surface of the liquid. If the readings in a dark-colored liquid are always made in the same way, the resulting error will not be great, and successive readings will be comparable.