1848.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



asi 



Decomposing power of water. 



On the Decomposing Pou-erof Water at High Tnnijeratiires. By 

 HicHABii TiLGHMAN. (Read before the American Philosopliical 

 Society, Philadelphia. IFi-om the Fraiihlin Journal.'] 



It has long been noticed, that partial decomposition is often 

 pRected in attempting to render anhydrous, by heat, certain salts 

 which require a comparatively hijjh temperature for the expulsion 

 of their watery crystallization. This effect is not limited to those 

 salts which are capable of decomposition by the action of heat 

 alone, but extends to many which, when previously rendered an- 

 hydrous, are entirely unaffected by this agent. The chloride of 

 magnesium offers a striking instance of such an action, being 

 almost entirely reduced to magnesia, with escape of hydrochbn-ic 

 acid, when its solution is evaporated by a strong heat ; the anhy- 

 drous chloride, when obtained by other processes, is, on the con- 

 trary, unaffected by the highest heat. 



Even chloride of calcium, a salt of much stronger radical base, 

 has been observed to give off a portion of acid, when all its water 

 •of crystallization is driven off by a red heat. In these and many 

 other instances, it seems evident that the escaping water of the 

 salt is the actual decomposing agent, and that the intensity of its 

 action depends solely upon the degree of heat whicli the salt can 

 sustain before giving it off. 



Contact of tlie salt and water, at high temperatures, appears to 

 be the only requisite of decomposition. It was therefoie thought 

 probable, that by exposing the salt, even in its aniiydrous state, t« 

 a high heat, and passing over it a current of aqueous vaj)0ur, 

 raised to a similar temperature, not only might the above-men- 

 tioned salts be completely decomposed, but also that many others 

 which have hitherto given no such signs of partial decomposition, 

 might be acted upon in a similar manner. 



On making the experiment, it was found, that not only the an- 

 hydrous chloride of calcium, but also the chlorides of strontium 

 and barium could be rapidly decomposed by exposing them, at a 

 high red heat, to a current of steam ; hydrochloric acid was copi- 

 ously evolved, and escaped along with the excess of ste.'im, while 

 the bases of the respective salts wci-e left in a free state ; the lime 

 remaining anhydrous frt.m the intensity of the beat employed, 

 while the baryta and strontia combined with a portiun of aqueous 

 Tapour, and were found in a state of hydrates. 



In these haloid salts, it is to be observed, that the addition of 

 the elements of water is absolutely essential to the decomposition ; 

 as neither the hydrogen which is contained in the acid, nor the 

 oxygen in the base, existed in the anhydrous salt. The action is, 

 therefore, the result of a double decomposition between the steam 

 and the chloride, as well as of the affinity of the liberated acid and 

 bai-e for water. 



The oxysalts, and the sulphates of magnesia, lime, strontia, and 

 baryta, unlike the haloid salts just mentioned, contain, even in the 

 anhydrous state, all the elements generally considered necessary 

 for the separate existence of the acid and "oases of which they are 

 composed. The application of the strongest heats to these salts, 

 causes, however, no liberation of their acid ; but, as with the chlo- 

 rides, this effect is immediately produced by the passage of a cur- 

 rent of steam over them at a high temperature, the baryta and 

 strontia being left in the state of hydrates, aiwl the other bases an- 

 hydrous. 



The intensity of the affinity between the acid and base of the 

 respective salts, is curiously illustrated by the gradual increase of 

 the heat necessary for their decomposition by the aqueous vapour. 

 Thus the sulphate of magnesia gives off its acid to the current 

 of steam at a low red heat, and consequently a large portion of the 

 acid may be condensed in an undecomposed state The sul|)hate 

 of lime requires a high red heat for its decomposition, and on this 

 account the greater part of its acid is resolved into snljihureous 

 acid and oxygen gas. The decomposition of the sulphates of 

 strontia and baryta, requires progi'essively higher heats, which, in 

 the case of the last salt, must be raised even to low whiteness. 



The subphosphate of lime, as it contains an acid much less vola- 

 tile than sulphuric, combined with an excess of a powerful base, 

 which adds to its stability, was selected as one of the most difficult 

 tests of this decomposing power of aqueous vapour: by a full 

 white heat, however, its phosphoric acid was slowly disengaged. 

 This phosphoric acid gave a white precipitate with nitrate of 

 silver, showing that its liberation and subsequent condensation in 

 contact with a great excess of aqueous vapour, liad not prevented 

 that c^iange which heat is known to produce upon this acid. 



It might he expected from the decomposition of the salts of 

 baryta, that' tlie sulphates and muriates of potash and soda would 



undergo the same change with even greater facility. But it wa? 

 found by experiment, tliat although the decomposition of these 

 last salts commenced with facility, w lien they were exposed to 

 steam at a red heat, yet the proportion of alkali thus liberated, 

 never exceeded a very small per centage of the residual salt, how- 

 ever long the operation might be contained. Attributing this pe- 

 culiarity to the volatile nature of the liberated hydrates of potash 

 and soda at high temperatures, siibstances capable of forming non- 

 volatile combinations with the alkalies were mixed with their salts, 

 previously to subjecting them to the action of the ste;un ; the 

 acids were then found to be completely disengaged w ith facility. 

 The fact that both lime and magnesia, substances capable of form- 

 ing chemical combinations of but the most feeble character witli 

 potash and soda, were found to produce the above effect, was con- 

 sidered as confirming, in a great measure, the hypothesis that tlie 

 volatility of their hydrates was the cause of the apparent dfficuUy 

 of completely decomposing the salts of these alkalies. 



The subphosphates and subsilicates of lime, baryta, and strontia, 

 act in the same manner as lime and magnesia, and in all these 

 cases the chemical combination is so feeble that, when cold, tlie 

 alkali is disengaged by the solvent powers of water alone. 



Alumina, which possesses so much of the acid character with re- 

 spect to the strong bases, is proportionably more efficient than any 

 of the preceding substances in aiding the decomposition of the 

 alkaline salts : it remains in combination with the alkali, when 

 cold, as a soluble ali;minate; but is eai^ily precipitated from its 

 solution by a current of carbonic acid gas. 



The calcination of potash alum leaves a mixture of alumina 

 and sulphate of potash, which Berthier has long since stated to 

 be converted into aluminate of potash by the continued action of 

 heat alone, the suljihuric acid being expelled fr( m the potash by 

 the superior affinity of the alumina at a high temperature. By 

 several careful repetitions of his experiment, in which the acci- 

 dental presence of aqueous vapours was entirely prevented, no de- 

 com])osition of this kind could be effected, even at a white heat. But 

 by the contact of aqueous vapour, produced by the combustion of 

 the fuel or otherwise, even in i-mall (luantity, and at much lower 

 temperatures, the decomposition is rapidly produced. It therefore 

 seems probable that the accidental contact of aqueous vapour was 

 the actual but unnoticed cause of the decomposition in Berthier'i! 

 experiment. 



The poweiful action of aqueous vapour upon anhydrous alum at 

 a high temperature, suggested the possibility that a similar action 

 might take place upon its mineral rejiresentative, the double sili- 

 cate of alumina and potash, or common felspar. It will be re- 

 membered that this salt, by the simple substitution of sulphuric 

 for the silicic acid which it contains, would be converted into an- 

 hydrous alum. To the action of heat alone, felspar presents this 

 difference from alum, that the silicate of alumina is as unaffected 

 by it as the silicate of potash itself; so that to produce an effect 

 upon felspar analogous to that upon alum, the silicic acid of both 

 the silicate of alumina and of the silicate of potash would have to 

 be removed. Silicic acid, in a free state, having been long knowu 

 to be slightly volatile in aqueous vapour at high temperatures, it 

 was thought that, in the present case, it might, like the other acids, 

 he disengaged even from a state of chemical combination, by the 

 same agents. Steam was therefore passed slowly, for some time, 

 over small fragments of highly-heated felspar. Beyond partial 

 fusion, no other visible change than a considerable degi-ee of vesi- 

 cularity in the parts most exposed was produced. These frag- 

 ments, heing finely pulverised and lioiled in water, the concentrated 

 solution vras strongly alkaline, and proved, by the usual tests, to 

 consist of aluminate of potash. 



After water ceases to extract aluminate of potash from the 

 powdered mineral, dilute sulphuric acid will produce from the 

 residue a small portion of alum. The actual analogy between 

 alum and felspar, substances so distinct in their origin and general 

 properties, yet differing only in the nature of their respective 

 acids, is reiidered still more striking by both thus yielding the 

 same product, when deprived of their acids by the same agent. It 

 is worthy of remark, tiiat, altliough the actual contact of the 

 steam in this experiment is confined to the mere surface of the 

 small fragments of felspar, yet the chemical decomposition pro- 

 duced by it is not confined to that surface, but spreads 'by a 

 " cementation action," through their entire mass : piJverization is, 

 therefore, required to obtain evidence of the internal change which 

 has been produced. 



All the experiments, so far made, would indicate that the follow- 

 ing was the general rule applicable to all salts^apable of sustaining 

 heat alone w ithout decomposition : — 



Whenever a salt, from it.; o\\ n elements alone, or by the adii- 



