168 



DEFINITION OF TERMS. 



all the crystallizable ingredients have 

 been separated by concentration, is heat- 

 ed to about 230° Fahr., poured into a 

 lirge stone-ware basin, and saturated with 

 diluted sulphuric acid. When cold, the 

 liquor is filtrated through woollen cloth, 

 and to every 12 oz. (apothecaries' mea- 

 sure) of it, is added 1000 grains of black 

 oxide of manganese in powder. The 

 mixture is put into a glass globe, or large 

 matrass with a wide neck, over which a 

 glass globe is inverted, and heat is ap- 

 plied, which causes the iodine to sublime 

 copious!}', and to condense in the upper 

 vessel. As soon as the balloon becomes 

 warm, another is substituted for it; and 

 when the second becomes heated, the 

 first is again applied. The iodine is 

 withdrawn from the globes by a little 

 warm water, which dissolves it very 

 sparingl}', and it is purified by undergo- 

 ing a second sublimation. The test made 

 use of for the detection of iodine in any 

 solution, when it is suspected to be pre- 

 sent, is starch, with which iodine has the 

 property of uniting, and of forming with 

 It a compound, insoluble in cold water, 

 which is recognised with certainty by its 

 deep blue color. The solution should be 

 cold at the time of adding the starch; and 

 if the color does not become apparent 

 simply on the addition of the starch, a 

 few drops of sulphuric acid should be 

 cautiously added, when, if any iodine is 

 present, the blue color will make its ap- 

 pearance. This test is so exceedingly 

 delicate, that a liquid containing -^jo\ooq 

 of its weight of iodine, receives a blue 

 tinge from a solution of starch. — Iodine 

 has a powerful affinity for hydrogen, 

 which it takes from animal and vegetable 

 substances in the sanie manner as chlo- 

 rine, and uniting with it iorms hydriodic 

 acid. 



The following are the methods for ob- 

 taining this acid in the gaseous and in the 

 liquid state: Into a flask to which a re- 

 curved tube is fitted, dipping under ajar 

 of mercury, are introduced eight parts of 

 iodine and one of phosphorus, and to the 

 mixture a few drops of water are added; 

 the water is immediately decomposed, 

 the phosphorus, seizing the oxygen, forms 

 phosphoric acid, while the hydrogen com 

 bines with the iodine 



water present in sufficient quantity to 

 dissolve the hydriodic acid, it passes over 

 in the gaseous state, and is collected over 

 the mercury. In contact with air it 

 smokes or fumes like the muriatic acid, 

 and, like it, reddens vegetable blues. It 

 is distinguished, however, from that acid, 

 by the superior affinity possessed by 

 chlorine for hydiogen, in consequence of 

 which, if chlorine and hydriodic acid 

 gases are mingled together, the yellow 

 color of the former disappears, and the 

 violet vapor of iodine make? its ap- 

 pearance, which proves the decom- 

 position of the hydriodic acid by the 

 chlorine. If the decomposition is com- 

 plete, the vessel will be wholly occu- 

 pied by muriatic acid gas. To obtain the 

 hydriodic acid in a liquid state, we have 

 only to conduct the gas through water, 

 until it is fully charged with it; or it may 

 be obtained by transmitting a current of 

 sulphuretted hydrogen gas through water 

 in which iodine, in fine powder, is sus- 

 pended. The iodine, from a greater affi- 

 nity for hydrogen than the sulphur pos- 

 sesses, decon)poses the sulphuretted hy- 

 drogen; and hence sulphur is set free, 

 and hydriodic acid produced. The con- 

 stitution of hydriodic acid i.s. 



By volume. By weight. 

 Iodine, - 50 - 124 



Hydrogen, - 50 - 1 



100 125 



The solution of hydriodic acid is easily 

 decomposed. Thus, on exposure for a 

 few hours to the air, the oxygen of the 

 atmosphere forms water with the hydro- 

 gen of the acid, and liberates the iodine. 

 Nitric and sulphuric acids likewise de- 

 compose it by yielding oxygen, the for- 

 mer being converted into nitrous, and the 

 latter into sulphurous acid. The free 

 iodine becomes obvious on the applica- 

 tion of the above mentioned test; the com- 

 pounds of hydriodic acid with the solifia- 

 ble bases may be easily formed, either by 

 direct combination, or by acting on the 

 basis in water with iodine. Sulphurous 

 and muriatic acids, as well as sulphuret- 

 ted hydrogen, produce no change on the 

 hydriodates, at the usual temperature of 

 the air; but chlorine, nitre, and concen- 

 As there is not'trated sulphuric acid, instantly decom- 



