874 HYDROGEN 



this precaution, owing probably to small traces of the sulphuric acid being carried 

 oVor during the distillation. 



It is quite impossible to conduct the operation so as to yield a product of uniform 

 strength ; it is absolutely necessary, therefore, to determine the percentage of real 

 hydrocyanic acid, and dilute it to the required degree. It fortunately happens that 

 1 grain of hydrocyanic acid yields almost exactly 5 grains of cyanide of silver; for 

 one equivalent of acid = 27 produces 1 equivalent of cyanide of silver=134; so that 

 27 I 134i:l : 4'96. The acid produced will have, probably, to be reduced to one of 

 two standards ; namely, the so-called Scheele's strength, containing 5 per cent, of acid, 

 or the P.L., containing 2 percent.; 100 grains of the former should, consequent I v. 

 3'ield 25 grains, and 100 of the P.L. 10 grains of cyanide of silver. In either < 

 calculation becomes obvious. 



2. The anhydrous acid. Several processes for conducting this dangerous operation 

 are known ; the following is, perhaps, the most generally convenient. A large glass 

 retort is so arranged that its neck is directed upwards at an angle of about 45 ; a 

 cork fitted to the aperture in the neck connects a glass tube with a bottle containing 

 a little chloride of calcium. From the latter vessel another tube proceeds to a U-tube 

 containing fragments of chloride of calcium, and from the latter a third, conducting 

 the dehydrated vapour of prussic acid to an upright glass tube contained in a mixture 

 of ice and salt. Into the retort is placed a mixture of 10 parts of yellow prussiate of 

 potash, 7 of oil of vitriol, and 14 of water. The retort is to be heated with a charcoal 

 fire, and the temperature of the bottle and U-tube, containing the chloride of calcium, 

 is not to be allowed to fall below 90, in order to prevent condensation of the anhy- 

 drous prussic acid taking place anywhere except in the tube contained in the freezing 

 mixture. The vapour of anhydrous prussic acid is so dangerous that the greatest 

 precaution must be taken to prevent inhaling the smallest portion. 



Detection of prussic acid. When prussic acid exists in moderate quantity in a 

 solution it may be detected by first adding a few drops of potash, then a mixture of 

 protosulphate and persulphate of iron, and finally a little hydrochloric acid ; a bright 

 blue precipitate indicates the presence of the acid. A much more delicate test, and 

 one that is applicable when, from the dilution of the solution, the salts of iron are no 

 longer capable of acting, is by the conversion of the prussic acid into sulphocyanide 

 of ammonium. For this purpose the prussic acid is to be warmed on a watch-glass 

 with a drop of sulphide of ammonium, until the solution has become colourless. The 

 addition of a trace of a solution of a persalt of iron -will show, by the formation of a 

 blood-red colour, the presence of the acid sought. A very neat mode of applying 

 this test is to place one drop of sulphide of ammonium on a watch-glass inverted over 

 another containing the suspected fluid. On leaving the apparatus in a warm place, 

 arranged in this manner, for a short time, the upper glass will bo found to contain 

 sulphocyanide of ammonium, which, after drying, will be in a state well adapted for 

 showing the reaction with a persalt of iron. C.G-.W. 



HYDRODYNAMICS. The mechanical science which treats of the motion of 

 fluids. This science has, of course, most important bearings on the pumping-engines. 

 water-wheels, &c., employed to facilitate the operation of the miner. It is not, how- 

 ever, possible to embrace this, which belongs to mechanical engineering, in this work. 

 See HYDRAULIC MACHINEBY, FOB MINES. 



HYDRO-EXTRACTOR. A name sometimes given to the machines employed 

 for expelling the water from woven goods. See DESICCATION. 



HYDROFLUORIC ACID. It was observed by Schwankhardt, in 1070, thatfluor 

 spar and oil of vitriol would eat into glass. Scheelo, in 1771, determined that this 

 peculiar property was due to the liberation of an acid from the fluor spar. 



Hydrofluoric acid is best obtained by placing finely-powdered fluor spar in a leaden 

 retort, and twice its weight of highly-concentrated oil of vitriol. By a gentle heat 

 the gas is distilled over, which must bo collected in a leaden tube, in which, by means 

 of a freezing mixture, it may be condensed into a liquid. If a solution of this acid in 

 water is required, the extremity of the tube from the retort is carried into water. 



Hydrofluoric acid attacks glass with great readiness, by acting on its silica. 



Grlass upon which any design is to bo el died, is covered with an etching-wax, and 

 the design made in the usual manner ; this is placed over a leaden vessel, in which 

 is a mixture of fluor spar and oil of vitriol ; a gentle heat being applied, hydrofluoric 

 acid escapes, and immediately attacks the gla?s. See FLUOHIM:. 



HYDROGEN 1 . (Hydrogens, Fr. ; Wasserstoff, Ger.) A permanently gaseous ele- 

 mentary body, the lightest of all known substan.-: I, its specific irravily being -0693 ; 

 100 cubic inches weighing, under ordinary pressure and temperature, only 2'14 grains. 

 It is therefore nearly 14'f> times lighter than atmospheric air. 



From its extreme lightness it was formerly used for filling balloons, but it has l>e.-n 

 superseded for that purpose by ordinary coal-gas, which can bo obtained at a much 



