FUJORIJTB HYDROGEN. ] 



CHEMISTRY. 



319 



animal substances. It is soluble in water and alcohol in 

 almost any proportion ; and being highly volatile, must be 

 kept in well-stoppered bottles. Its symbol is Br, aud 

 the equivalent=80'0. 



Combined with metals and other bases, it forms tlie 

 substances called bromides. Like iodine, its chief use is 

 in connection with photography, in which it has been em- 

 ployed in the state of vapour, to increase the sensitiveness 

 of the iodide of silver on Daguerreotype plates ; and, in 

 the form of bromide of potassium, it is considerably used 

 in paper-printing processes. 



It is prepared from hydrobromic acid, which is a com- 

 bination of hydrogen and bromine ; symbol, HBr ; equi- 

 valent, 81. This acid is analogous in its constitution 

 to the hydrochloric, hydriodic, and hydrofluoric. Its 

 preparation is somewhat difficult ; bromide of phosphorus 

 being decomposed by means of water, when phosphoric 

 acid and the hydrobromic are produced. 



Bromic acid is a compound of bromine with oxygen, 

 in the proportion of one equivalent of the former to five 

 of the latter. Its symbol is Br, Oj ; equivalent = 120. 

 Wn shall describe other combinations of bromine tinder 

 the head of their bases. 



FLUORINE. 



Fi i 'RiXE is a substance which has but little commercial 

 value, and is never met with in a free state. It is obtained 

 Fluor or Derbyshire spar. Its symbol is Fl, and the 

 equivn]ent = 18'9. Its most known combination is that of 

 hydro'luoric acid, in which it is combined with hydrogen. 



Hydrofluoric acid may be readily obtained by heating 

 some coarsely pounded fluor spar, together with strong 

 sulphuric acid, in a leaden vessel. A glass one cannot be 

 used, because tne acid acts powerfully on that substance. 

 The fumes of the acid and the liquid should be carefully 

 avoided, as they produce the most painful wounds if they 

 come in contact with the skin. 



Hydrofluoric acid, besides its occasional use in the la- 

 boratory, is much employed for etching glass ; and the 

 method of conducting this process may be seen in the 

 following experiment. 



Ej-)ifriinent 31. Make a tray of lead by bending up 

 tho sides of a sheet of that metal so as to form a riiu on 

 all sides. Support this tray on a stand, and throw inside 

 it some coarsely pounded fluor spar, to which some strong 

 sulphuric acid may be added. Cover a glass plate with 

 beeswax ; and on this, when cold, make any sketch by 

 means of a needle-point, so as to expose the glass surface. 

 Then place the plate, with the sketched part face down- 

 wards, over the mixture of fluor spar and acid last 

 mentioned, so that its edges may rest on the edges of the 

 tray. Apply a gentle heat by means of a spirit-lamp, 

 when the hydrofluoric acid will rise in vapour and attack 

 the exposed surface of the glass. By these means such 

 portions will be completely etched out ; and on the wax 

 being removed, the design will appear beautifully traced 

 on the glass surface. 



The best wax for covering the glass is that obtained by 

 melting wax candle-ends. It should be run over both 

 sides of the glass, lest the vapour of the acid might 

 attack both sides. Some pretty effects may be produced 

 by first sketching the design, with a soft black-lead pencil, 

 Fig. 42. 



on paper. This drawing is easily transferred to the wax 



surface by pressing the pencilled paper thereon. This 

 will form a guide for tne subsequent tracing by the 

 needle-point. By such means the figures are produced 

 on the ornamental sheets of glass so much used for 

 sashes, in place of Venetian blinds. The preceding figure 

 illustrates the mode of carrying out the experiment we 

 have been describing. 



HYDROGEN. 



NEXT to oxygen, hydrogen stands of the greatest im- 

 portance in the list of elements. By the combination 

 of the two, we have the water of the ocean, of our 

 rivers, <tc. Combined with carbon, hydrogen enters 

 into the composition of most vegetable and animal sub- 

 stances : as carburetted hydrogen, a product from coals, 

 it is extensively used as fuel and for illuminating pur- 

 poses ; and the same results are obtained from vegetable 

 and animal oils, through its presence in them. Its 

 other numerous compounds will be noticed as we 

 proceed. 



Hydrogen is the lightest known body in nature, its 

 specific gravity being OH)69 ; that of air being I'OOO ; it 

 therefore weighs but a little over one-fifteenth part of 

 its bulk of atmospheric air. For this reason it has been 

 employed to fill balloons, both in its pure aud com- 

 pound states. 



Hydrogen maybe procured, in various shades of purity, 

 from water, coal, oils, <fcc. The following experiments 

 will afford instances of the kind. 



Experiment 32. Pour some dilute sulphuric acid on 

 some iron nails contained in a saucer or evaporating 

 dish. Large bubbles of gas will speedily arise ; and if a 

 lighted match be brought near them, they will at once 

 ignite. 



Experiment 33. Put a few iron nails, or some shavings 

 of zinc, into a glass bottle, and fit into its neck a cork 

 containing a glass tube, one of the edges of which has 

 been drawn to a jet. Add some dilute sulphuric ac,d 

 to the metal, and fit in the tube. Gas Fig. 43. 



will be given off; but this should be 

 allowed to escape, because, being mixed 

 with air, it would explode, and perhaps 

 burst the bottle if the gas were ignited. 

 After sufficient time lias elapsed to re- 

 move all the air from the bottle, apply a 

 light to the jet, when the gas will burn 

 with a yellowish-coloured flame. The an- 

 nexed figure shows the kind of bottle re- 

 quired for this experiment. 



When a considerable quantity of hy- 

 drogen is required for the purposes of experiment, a 

 bottle with a bent tube attached to it, such as is repre- 

 sented in the annexed cngrav- Fig. 44. 

 ing, may be employed ; and the 

 gas is to be collected by the 

 pneumatic trough, in a similar 

 manner to that mentioned for 

 oxygen. It must, however, be 

 borne in mind, that hydrogen 

 is so much lighter than air, that 

 it will at once escape if the 

 vessel containing it be held 

 mouth upwards. The following experiments, illus- 

 trating the levity of the gas, may impress the fact on 

 the memory of the student. 



Experimen' 34. Hold a glass vessel with its mouth 

 over the jet of the bottle described in Experiment 33. 

 The gas will rise into the vessel, and so expel the air. 

 This may easily be proved to have taken place by apply- 

 ing a light to the mouth of the vessel, when the gas 

 will at once catch fire. 



Experiment 35. Fill a glass vessel, as in the last ex- 

 periment, with hydrogen, and then bring near to it 

 another vessel, also with its mouth downwards. Incline 

 that already filled with gas beneath the mouth of that 

 filled with air, and gradually allow the gas to pass from 

 one vessel to the other, in the way illustrated in the 

 following engraving (Fig. 45). That the gas is really 



