82 



THE POPULAE EDUCATOR. 



Iodine gives off vapour at all temperatures, and must there- 

 fore be kept in a bottle with a glass stopper, as the vapours 

 attack cork. The skin and organic bodies are stained by it 

 yellow. This stain, however, passes away so soon as the iodine 

 evaporates. It is slightly soluble in water, this liquid being 

 capable of holding yJ^ of its weight in solution. It is freely 

 dissolved in alcohol and ether, and a minute piece of iodine im- 

 parts a rich colour to bisulphide of carbon ; but the chief liquid em- 

 ployed for its solution is the solu- 

 tion of a soluble iodide, such as 

 potassium iodide. 



The most delicate test for iodine 

 is the intense blue colour it imparts 

 to starch. However, to effect this, 

 the iodine must be in an uncom- 



bined state. Chlorine water or Fig 



nitric acid will always liberate 



iodine from its combinations, and therefore, if the presence of an 

 iodide be suspected, add one or other of these agents to the 

 solution, and then the starch paste. One part of iodine, dissolved 

 in a million parts of water, will be made apparent by this test. 



Hydriodic Acid (symbol, HI ; combining weight, 128 ; density, 

 64). This acid is best prepared in a manner similar to that by 

 which hydrobromic acid was procured namely, by acting on 

 phosphoric iodide with water, thus 



PI, + 3H a O = 



+ SHI. 



However, it may be made directly by heating iodine in an atmo- 

 sphere of hydrogen ; or a solution of this acid may be easily 

 prepared by suspending iodine in water, and transmitting a 

 current of sulphuretted hydrogen gas until the brown colour 

 disappears. Sulphur is deposited, and hydriodic acid formed, 

 which goes into solution in the water. If this solution be 

 exposed to the air in sunlight, it gradually absorbs oxygen, 

 the hydrogen of the acid forming with it water, and the libe- 

 rated iodine renders the liquid brown. Iodides are formed by re- 

 placing the hydrogen of the above acid by the metal, according 

 to its atomicity. When iodides are heated, the iodine goes off, 

 and an oxide of the metal is formed. Of course, in the case of 

 the noble metals (An, Ag, Pt, and Hg), the metal remains. The 

 elements, chlorine and bromine, when acting on iodides, have the 

 power to remove the iodine, and insert themselves in its place. 



Oxides of Iodine. This element 

 has a greater affinity for oxy- 

 gon than either of the preceding 

 halogens; but of its oxides, the 

 only two which have been studied 

 are iodic acid and periodic acid. 

 Iodic acid, or Jiydnc-iodate (sym- 

 bol, HIO 3 ), corresponds closely to 

 chloric acid. It is prepared by 

 the action of strong nitric acid on 

 iodine. When the iodine has nearly 

 disappeared, the liquid upon cool- 

 ing gives crystals of iodic acid. 



When iodine is dissolved in 

 caustic potash or soda, the re- 

 sult is a mixture of iodide and 

 iodato of potassium or sodium. 

 The iodate being much the most 

 difficult to dissolve, may easily 

 be separated from the iodide. 

 Or if, in the course of the pro- 

 cess, chlorine gas be passed, then no iodide is formed ; thus 



I + 6KHO + 5C1 = KIO, + 5KC1 + 3H a O. 



When iodates are heated, they behave like chlorates, giving off 

 oxygen. 



Iodic acid is at once decomposed by sulphurous acid. This 

 provides a test for the presence of sulphur in any combustion. 

 Soak a piece of paper in a mixture of potassium iodate and 

 starch-paste, then expose it to the fumea ; if any sulphurous 

 acid be present, the paper becomes blue. Morphia possesses a 

 like power, and hence by this test the presence of this powerful 

 poison may be detected. 



Periodic Acid, or Hydric-psriodate (HIO 4 ). This acid can 

 only be obtained in combination. Sodium per-iodate (NaIO 4 ) 

 may be prepared by passing chlorine through a solution 



of three atoms of caustic soda, and one atom of sodium 

 iodate. 



Nitric iodide, which is generally supposed to be the ter-iodide of 

 nitrogen (NI 3 ), is an interesting compound, from the readiness 

 with which it explodes. Place a little iodine in a capsule, and 

 pour upon it sufficient ammonia to cover it well. Allow this to 

 digest for half an hour ; then pour off the supernatant liquid, 

 and place the brown substance upon pieces of blotting paper -a 

 little on each paper; leave them 

 to dry, if by a fire, at some distance 

 from it. When dry, a shake of 

 the paper is sufficient to determine 

 the decomposition with explosion. 

 Iodine is noted in the medical 

 world for its great powers of 



44. absorption. Glandular swellings 



may be removed by it which have 



resisted every other means. It is used for this purpose in a solu- 

 tion, which is made as above described. Its action is greatly ac- 

 celerated if a few grains of potassium iodide be taken internally 

 each day. FLUOBINE. 



SYMBOL, F ATOMIC WEIGHT, 19. 



Hitherto no attempt to isolate this element has been success- 

 ful. Its affinities are so powerful, and its action on the human 

 frame so violent, that little is known of it. Its only compound 

 which occurs in any abundance, is Derbyshire spar, calcium 

 fluoride (CaF 2 ). Many minerals contain this salt in small 

 quantities. It is detected in teeth, and even in the blood of 

 animals. Fluorine is not known to combine with oxygen, nitrogen, 

 sulphur, or the other halogens. 



Hydrofluoric Acid (symbol, HF ; combining weight, 20 ; den- 

 sity, 10). To prepare this acid, Derbyshire or fluor spar is 

 reduced to a powder, introduced into a leaden or platinum retort 

 (Fig. 45), and then mixed with sulphuric acid. Upon the 

 application of heat, this reaction ensues 



H 3 SO + CaF a = CaSO, + 2HF. 



The bent part of the tube in Fig. 45 is immersed in a freezing 

 mixture, and here the hydrofluoric acid condenses into a colour- 

 less liquid. 



It is an energetic acid, and has the power of converting 

 metallic oxides into water and metallic fluorides. Of all chemical 

 substances, its effect on the skin 

 is the most painful. It will pro- 

 duce a sore which exhibits but 

 small inclination to heal. Its most 

 characteristic property is its power 

 to etch on glass. It effects this, 

 because with silica one of the 

 constituents of glass it forms a 

 gaseous product (Si F 4 ), hydrofluo- 

 silicic acid : thus 



SiO, + 4HF == 2H,O + SiF. 



To exhibit its effects, a glass 

 plate is covered with bees'-wax, 

 upon which fluorine has no action, 

 and any design traced with a sharp 

 point in the wax. This is exposed 

 to the vapour of hydrofluoric acid, 

 and the parts of the glass exposed 



Fig. 45. aro etched ; the glass is " frosted" 



by the vapour; but if the solution 



of the acid, which is sold in gutta-percha bottles, be poured on 

 glass, the glass is eaten away. Any photographic artist will 

 at once appreciate this fact to enable him to remove the frost- 

 ing from the back of the glass stereoscopic slides, and thus it 

 will be possible to take " prints " from them. 



The halogens form the best defined of natural groups of ele- 

 ments. Their atomic weights are almost in arithmetical pro- 

 gression. 



F = 19. 

 Cl = 35-5. 

 Br = 80. 

 I = 127. 



Chlorine is a gas, bromine a liquid, and iodine a solid. More- 

 over, bromine, in its affinities, is a mean between chlorine and 

 iodine, and all form directly, with the metals, salts. 



