glass tubing is taken by the glass-blower and sealed at one end. A bulb is then formed for the reception of the ballast, and the upper end is drawn out into a smaller tube. Mercury or shot is then added until the instrument floats in an upright position. It is placed in pure distilled water at 60° Fahrenheit. If the hydrometer is to be used for measuring the specific gravity of liquids heavier than water, it is loaded until the level of the water almost reaches the top of the tube. The instrument is then placed in a second heavier liquid of known specific gravity. It will come to rest farther out of the fluid than before, since it must needs displace less in order to float. This second point established, it is easy to construct the scale. If the hydrometer is to be used for liquids lighter than water, let us say for alcohol, it is so loaded that when placed in pure water the level will only reach up to the lower part of the tube. It is then placed in a lighter liquid of known specific gravity. It will sink lower in this case, since it must needs displace more of the fluid in order to float. This second point established, it is an easy matter to continue the graduation upward in space and downward numerically. The scale employed depends upon circumstances. In the direct-reading hydrometers, the point to which the instrument sinks in pure water is marked one, and the other readings express directly the specific gravity of the fluid into which it is plunged. In others, the scale is empirical—that is to say, the degrees bear no relation to actual specific gravities. In certain manufacturing processes such scales are used with the purpose of keeping trade secrets. Where the hydrometer is for a special use, such as measuring the specific gravity of alcohol, it is known as an alcoholometer, and the marking ascends from pure water at the bottom of the graduated tube to pure alcohol at the top. The degrees give at once the percentage of alcohol in the liquid under examination. One of the most familiar special forms is the lactometer, the hydrometer used for measuring the specific gravity of milk. The scale is commonly drawn on a piece of paper which is fastened inside the tube in the right position. The end is then sealed before the blowpipe, and the instrument is ready for use.
The manufacture of pressure-gauges and other glass instruments for measurement proceeds in much the same fashion.
In chemical and physical laboratories the use of glass instruments is a simple necessity. Combustion-tubes, beakers, funnels, test-tubes, watch-crystals, burettes, pipettes, absorption bulbs, bell-jars, flasks, apparatus for electrolytic decompositions, and a hundred other essential articles could scarcely be made of any other material. Here the transparency of the glass, its great strength, and its almost total indifference to the action of reagents give it special suitableness. The principles involved in the manufact-
