GLA SS-MAKING. 6 3 1 



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 00 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 disj)lace less in order to float. This second point es- 

 tablished, 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 numeri- 

 cally. 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 meas- 

 uring the specific gravity of alcohol, it is known as an alcohol- 

 ometer, 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 exami- 

 nation. 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 instru- 

 ments for measurement proceeds in much the same fashion. 



In chemical and physical laboratories the use of glass instru- 

 ments 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 hun- 

 dred 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- 



