Jdne 29, 1883.] 



♦ KNOWLEDGE ♦ 



?85 



replaces it. Then suddenly flashes into \-iew a rainbow- 

 tinted array of lines, multitudinous in number and most 

 beautiful in aspect (Mr. Lookyer od4Iy enough imagined 

 in 1870 that the absorption took place below the visible 

 surface, and therefore not only rejected the idea that they 

 would be seen, but for a long time denied that they had 

 been seen. However, repeated observations at last con- 

 vinced astronomers that the other view was the sounder 

 — and now these lines have even been photographed.) 



OUR CHEMISTRY COLUMN. 



By \Villi.\m J.\no, F.C.S. 

 THE HALOGKN.';. 



FROM the ordinary definition of an element as a sub- 

 stance which cannot be split up into two or more 

 different substances, it follows that no two elements can be 

 exactly alike, because they would then be one and the 

 same element But while wo cannot take two of the 

 elements and say that they are in every way similar, we 

 can nevertheless arrange them into groups, between the 

 members of which there is a strong family likeness. The 

 classification of elements in this manner has always been 

 a fascinating occupation to chemists, for if a group consists 

 of three elements — say. A, B, and C — which are remarkably 

 like one another, is it not possible that they may be 

 modifications of one and the same element of 

 simpler constitution than has hitherto been discovered ? 

 Of such groups, perhaps, none present such striking and 

 interesting resemblances as that whose name heads the 

 present article. The " halogens " include the three 

 elements — chlorine, bromine, and iodine, of whose pre- 

 sence in sea-water we dealt in the last paper. The group 

 is generally made to include another element — fluorine. 

 This latter, however, if the others be looked on as brethren, 

 cannot aspire to a much closer relationship than that of a 

 first cousin. Let us confine our attention to the first three : 

 their study can be best prosecuted by actually preparing 

 the substances and experimenting on them. The chemical 

 student who has a few flasks and other appurtenances 

 of tlie laboratory at hand, can easily prepare for himself a 

 few jars of chlorine. Let him, however, avoid so doing in 

 any place where the fumes of the gas will penetrate to 

 those who may not be enamoured of practical chemistry : 

 for, while sulphuretted hydrogen is known everywhere as 

 the gas with a. downright stench of rotten eggs, commend 

 us to chlorine for a gas making existence a burden in its 

 immediate neighbourhood. Its odour is most pungent and 

 irritating, and even a comparatively little, present in the 

 air, makes breathing diflicult ; still there is this comfort 

 about it, the fumes are not injurious to health otherwise 

 than by their irritating action on the mucous membrane. 

 If there is not sufficient of the gas disseminated through the 

 air to make breathing inconvenient, no harm is being done. 

 Curiously enough, chlorine and sulphuretted hydrogen, 

 before referred to as the gas of rotten-egg-like odour, 

 mutually destroy each other, producing inodorous sulphur 

 and far less disagreeable hydrochloric acid. So, a stranger 

 in a chemical laboratory, inconvenienced by the chlorine 

 being made by the worker on his right, may get rid of that 

 gas if he can only persuade some one else on his left to 

 make sulphuretted hydrogen in just the right proportion to 

 neutralise the chlorine. The writer, however, cannot from 

 personal experience recommend this method of purifying 

 the air of a laboratory or other room from chlorine ; some- 

 how or other the exact pointof proper quantities seems as diffi- 



cult to realise, as it is to concei^•e how those two disagreeable 

 niighbours, the Kilkenny cats, managed to render a service 

 to Kilkenny by mutually destroying each other, except the 

 tips of their tails. This property of chlorine is, neverthe- 

 less, a useful one to remember, for in the vicinity of 

 chemical works, where water impregnated with the gas is 

 allowed to enter the drains, it destroys the sulphuretted 

 hydrogen, and also is inimical to the life of disease germs. 

 However, despite the obstacle of smell, one may manage to 

 make the gas in either the open air (provided there is 

 no wind) or an outhouse. For this purpose a flask, 

 fitted with a cork and delivery tube, after the manner de- 

 scribed in text-books for the preparation of gases, will be 

 needed ; in this flask a mixture of sulphuric acid, common 

 salt, and manganese dioxide is placed, the flask is gently 

 heated and the gas collected in jars over warm water. A 

 littli' bromine and iodine may be readily purchased from 

 the chemist, provided the would-be purchaser of iodine can 

 persuade the vendor he is not a dynamiter in embryo. 

 Some bromide and iodide of potassium should also be 

 obtained. 



And now let us see what our three elements are like ; 

 chlorine is ■ a gas, bromine a liquid, and iodine a solid. 

 Here there is, you see, a sort of ascending the scale through 

 the three gradations of form in which matter is known to 

 exist. We find, too, that in passing from chlorine to 

 bromine, there is a deepening in colour ; the former is a 

 greenish yellow, the latter a deep red, and also, at ordinary 

 temperatures, gives oft' a vapour of the same tint ; iodine 

 is also dark in colour, but if a few grains are heated 

 in a test-tube or flask, a vapour of a superb violet 

 hue is produced. The whole three elements have a very 

 similar smell, but of the three, that of bromine is the 

 worst. The name itself is derived from the Greek bromos, 

 a stench. The three elements can exist in the state of 

 \apour, and if the vapours are weighed, the density of 

 chlorine is found to be 35 5 (hydrogen always being taken 

 as unity), that of bromine 80, and of iodine 127 ; here, 

 again, there is the same gradation. It may be noticed, in 

 passing, that the density of bromine is almost the mean of 

 those of chlorine and iodine. Powerful acids are formed by 

 the union of one atom of each of these elements with one 

 atom of hydrogen. These acids are all gaseous and all readily 

 soluble in water, producing in this manner pimgent fuming 

 liquids. The salts of these acids with the same metals 

 crystallize in the same shape, and are very similar in 

 appearance ; of these bodies common salt is a familiar ex- 

 ample. The name " halogen " is applied to members of this 

 group because of their thus forming salt-like bodies, the 

 word literally signifying "salt-producer." As the density 

 of the halogens increases, their chemical activity diminishes, 

 so that in most instances chlorine is the most powerful 

 element of the three. The two salts that have been 

 suggested for examination, bromide and iodide of potassium, 

 are both very soluble in water. Pour some of a solution 

 of the bromide into one of the jars of chlorine gas that 

 have been made, notice that the colour changes to deep 

 red, and if sufiicient bromide has been added, little red 

 drops of bromine will be seen. In the same way, 

 pour just a single drop of bromine in some of the iodide 

 solution, and iodine is liberated ; by adding some carbon 

 disulpliide, and shaking up, the iodine is dissolved by the 

 disulpliide, to which it imparts the same purple tint as 

 that characterising its vapour. These experiments illus- 

 trate the principle of the method employed for the extrac- 

 tion of these elements from sea-water. Chlorine is passed 

 through the " bittern " before described, and the free 

 bromine dissolved out by ether. Iodine is obtained by 

 burning the sea-weeds, which extract it from sea-water, and 



