HYDROCHINON DEVELOPER: See Photography.
HYDROGEN, AMALGAMS AS A SOURCE OF NASCENT: See Amalgams.
HYDROGEN PEROXIDE AS A PRESERVATIVE: See Preserving.
HYDROMETER AND ITS USE.
Fill the tall cylinder or test glass with the spirit to be tested and see that it is of the proper temperature (60° F.). Should the thermometer indicate a higher temperature wrap the cylinder in cloths which have been dipped in cold water until the temperature falls to the required degree. If too low a temperature is indicated, reverse the process, using warm instead of cold applications. When 60° is reached note the specific gravity on the floating hydrometer. Have the cylinder filled to the top and look across the top of the liquid at the mark on the hydrometer. This is to preclude an {402} incorrect reading by possible refraction in the glass cylinder.
HYGROMETERS AND HYGROSCOPES:
Paper Hygrometers.
| Cobalt chloride | 1 ounce |
| Sodium chloride | 1/2 ounce |
| Calcium chloride | 75 grains |
| Acacia | 1/4 ounce |
| Water | 3 ounces |
The amount of moisture in the atmosphere is roughly indicated by the changing color of the papers, as follows:
| Rose red | rain |
| Pale red | very moist |
| Bluish red | moist |
| Lavender blue | nearly dry |
| Blue | very dry |