156 



DEFINITION OF TERMS. 



Other, by canals or aqueducts, and to 

 elevate it by pumps, jets-d'eaux, and 

 other hydraulic engines, either for the 

 purpose of ornament or use. 



Hydro-carbonates, combinations of 

 carbon with hydrogen. A gas of this 

 name is obtained from moistened char- 

 coal by distillation. 



Hydrodynamics, treats of the state 

 and force of fluids at rest, or in motion, 

 whether liquids or gases. It is divided 

 into hydrostatics, hydraulics, pneumatics, 

 and acoustics, (see the separate articles.) 

 Ency. Jimer. 



Hydrogen, in chemistry, one of the 

 simple combustibles, the base of hydrogen 

 gas, formerly called inflammable air. 



Hydrogen gas. — To obtain this put 

 one part of iron filings into a retort, at- 

 tached to the pneumatic cistern, and pour 

 thereon two parts of sulphuric acid pre- 

 viously diluted with four times its bulk 

 of water. Immediately the mixture be- 

 gins to boil or effervesce with violence, 

 and air bubbles rush abundantly from the 

 beak of the retort. 



Allow them to escape for a little, till 

 you suppose that the common air which 

 previously filled the retort has been dis- 

 placed by the newly generated air. Then 

 place an inverted jar over the beak of the 

 retort. 



The bubbles rush in abundantly and 

 soon fill the jar. 



Hydrogenized-sulphurets,c&Y\,di'm bases 

 combined with sulphureted hydrogen. 



Hydrography, the art of measuring 

 and describing the sea, rivers, lakes, and 

 canals. 



Hydrometer, an instrument much used 

 for determining the specific gravities of 

 liquids. It usually consists of a hollow 

 sphere of glass or metal, and balanced by 

 another below, containing quick silver 

 or a metallic weight. The larger ball 

 has a shorter neck into which is screwed 

 a graduated stem which by a small weight 

 causes the instrument to descend in the 

 fluid with part of the stem. When this 

 instrument is swimming in a fluid the 

 part of the fluid displaced by it will be 

 equal in bulk to the part of the instru- 

 ment under water, and equal in weight 

 to the whole instrument. This instru- 



[ ment, therefore, marks the difference 

 in the density of fluids by the place at 

 which the surface cuts the stem as it 

 floats, or by the proportionate addition 

 or diminution of weight which is re- 

 quired to make it float at the same level 

 in each. 



The ureonieter is more simple and ac- 

 curate. A glass phial, about two inches 

 in diameter, and seven or eight long is 

 corked tight; into the cork is fixed a 

 straight wire one-twelfth of an inch in 

 diameter, and thirty inches long. The 

 phial is loaded with shot so as to sink in 

 the heaviest liquid, leaving the wire just 

 below the surface. The liquor is then 

 placed in a glass cylinder three or four 

 feet long, with a scale of equal parts on 

 the side, by which the point to which the 

 top of the wire sinks is marked. 



This instrument is so delicate that the 

 sun's rays falling upon it, will cause the 

 wire to sink several inches; and it will 

 rise again when carried into the shade. 



Hydro-oxides, metallic oxides com- 

 bined with water. 



Hydrostatics have for their object the 

 weight and pressure of fluids; and in this 

 branch of science the art of determining 

 the specific gravities of bodies is usually 

 included. Fluids like solids are impelled 

 by their gravity, and press upon every 

 thing that oppose their fall, but from 

 their nature they press in a diiferent 

 manner from solid bodies, hence arise 

 the peculiar phenomena concerning which 

 we are now to inquire. 1st, That the 

 parts of the same fluid act with respect 

 to their weight and pressure independent- 

 ly of each other. 2d, Fluids press equally 

 in all directions. 3d, All the parts of 

 the same fluid are in equilibrium with 

 each other, whether they are contained 

 in one vessel or many, provided they 

 communicate with each other ; and their 

 surfaces also are always in a plane parallel 

 to the horizon. A syphon draws liquid 

 from a vessel because the pressure of the 

 atmosphere is removed from the parts 

 entering the syphon, and so long as the 

 surface of liquid in the vessel is higher 

 than the discharging end of the syphon 

 it will continue to flow, but no longer, 

 because the equilibrium of atmospheric 



