HYGROMETER 875 



cheaper rate; the difference of buoyant power being compensated by increasing 

 considerably the size of the balloon. It is itself inflammable, but will not support 

 combustion. la burning it combines with oxygen, forming water, which contains |th 

 of its weight of hydrogen. 



It is generally prepared by the action of dilute sulphuric acid on zinc, although 

 there are many other processes which furnish it ; as the decomposition of steam by 

 iron fillings with the aid of heat, the decomposition of water by electricity, &c. 



In the act of combining with oxygen, as when burnt in the oxy hydrogen blow- 

 pipe, the greatest possible heat is obtained ; a piece of stout platinum wire being fused 

 when placed in the flame, which cannot be affected by the greatest heat of our 

 furnaces. 



Hydrogen is often used in the process of soldering metals ; it which case it is 

 requisite to bring the two surfaces of the metal together in a perfectly metallic state 

 at a high temperature. Hydrogen effects this completely ; by its combustion it 

 supplies the heat, and by entering into combination with the oxygen of the air, pre- 

 vents the formation of oxides, which are so easily formed at the temperature required 

 for the melting of the metals, and which, when present, prevent the union of the 

 surfaces. See AUTOGENOUS SOLDERING. 



Hydrogen is often used also for the reduction of metals from their different com- 

 binations ; the reduction is effected by passing a current of hydrogen over the com- 

 pounds heated to redness. 



HYDROGEN 1 , PHOSPHORETTED, or Phosphamine. A compound of hydro- 

 gen and phosphorus, PH 3 . It is spontaneously inflammable if it escapes into the 

 air. 



HYDROGEN. SULPHURETTED. See SULPHURETTED HYDROGEN. 



HYDROMETER. An instrument for ascertaining the specific gravities of liquids. 

 Baume's hydrometer, which is much used in France, and other countries of the con- 

 tinent of Europe, when plunged in pure water at the temperature of 58 Fah.r., marks 

 upon its scale ; in a solution containing 15 per cent, of common salt (chloride of 

 sodium), and 85 of -water by weight, it marks 15 ; so that each degree is meant to 

 indicate a density corresponding to one per cect. of that salt. See ALCOHOLMKTER. 



HYDROPHONE. A variety of opal which readily imbibes water, and when 

 immersed: it becomes transparent, though opaque when dry. It is found in Hungary 

 and in Ireland, near the Giant's Causeway, and at Crosreagh, Ballywillin. 



HYDROSTATIC BAX.ANCE. HYDROSTATIC PRESS. A balance or 

 a press constructed in obedience to the law that In a quantity of fluid submitted to 

 compression, the whole mass is equally affected, and similarly in all directions. The 

 subject belongs to Practical Engineering. 



HYDROSTATICS. The science which treats of the equilibrium of fluids, and 

 of the pressure exerted by them. 



In the engineering arrangements by which water is supplied to towns, hydrostatics 

 becomes of the utmost importance. The highest possible level is obtained for the 

 reservoir ; and from this a series of pipes is arranged through all the streets and 

 houses. The tendency of the water is to rise to its original level, and hence all the 

 pipes are filled with water, and in all such as are below the level of the water in the 

 reservoir a pressure upward is exerted equal to the height of the reservoir above 

 that point ; and if a hole is pierced in the pipe, the water jets out with a force equal 

 to this pressure. In the highest houses, the water perhaps only finds its level, and 

 flows out without pressure quickly. See HYDRAULIC CRANE ; HYDRAULIC MACHINERY, 

 FOR MINES. 



HYDROSULPHURETS or Hydrosulphides. Chemical compounds of bases with 

 sulphuretted hydrogen, or hydrosulphuric acid. 



HYDROXYIi. A radical containing HO 2 (HO). Great prominence has been 

 given to it in recent chemical theories. Peroxide of hydrogen, or oxygenated water, 

 may be regarded as free hydroxyl, since it contains the molecule, or two atoms, of 

 hydroxyl 2HO* (H 2 O 2 ). 



HYGROMETER (vyp6s,moist, fierpov, measure), is an instrument which measures 

 the amount or degree of moisture in the atmosphere. 



Water, both liquid and solid, always tends to assume the gaseous form, and in so 

 doing exerts an elastic force, increasing as the temperature rises, but in a higher ratio, 

 and in order that such increasing force may be exerted when water is confined in a 

 given space, it is necessary that a larger and larger quantity of water should become 

 vapour ; if the weight of water be not increased, its elastic force will rise with its 

 temperature, but in a much less ratio. For aqueous vapour, like any other gas, tends 

 to expand by ^th part of its volume at 32 Fahr. for every degree of increased 

 temperature. The terms saturated and of maximum elastic force are applied to 

 aqueous vapour, when any reduction of temperature or increase of pressure causes 



