periment, the reverse decomposition is possible, viz. carbonate of barytes being digested in solution of sulphate of potash, we obtain sulphate of barytes and carbonate of potash. Are we hence to infer, that sulphate of barytes and carbonate of potash having for some time amused the operator by the production of an alkaline sulphate and earthy carbonate, will change their mood, and retracing their steps, restore things to their pristine condition; and thus in alternate oscillation for ever? If chlorine gas be made to act on the oxides of mercury, tin, or antimony, it will unite to the metallic base, and displace every particle of the oxygen. Now, the resulting chlorides cannot owe their purity to any superiority of cohesive force which they possess over the oxides, which, on the contrary, are both denser and more fixed than the new compounds. Finally, if 25 parts of pure magnesia mixed with 35.6 of dry lime, be digested in 85 parts of nitric acid, sp. gr. 1.500, diluted with water, we shall find that the whole lime will be dissolved, but not a particle of the magnesia. On decanting the neutral calcareous nitrate, washing and drying the earthy residuum, we shall procure the 25 parts of magnesia unchanged. We are, therefore, entitled to affirm, that affinity is elective, acting in the different chemical bodies with gradations of attractive force, liable however to be modified, as we have shown in the case of muriate of lime and carbonate of ammonia, by temperature, and other adventitious powers. Decompositions which cannot be produced by single attractions, may be effected by double affinity; and that, we may expect with the greater certainty, a priori, if one of the two resulting compounds of the double interchange, naturally exists in the solid or aeriform state. And if the one resulting compound be solid and the other gaseous, then decomposition will be certain and complete. This applies with equal force to single affinities, or decompositions. Thus when sulphuric acid and muriate of lime in due proportions are exposed to heat, a perfect decomposition is accomplished, and pure sulphate of lime and muriatic acid gas are produced. But when the various mixed ingredients remain in solution, it is then reasonable to think with Berthollet, that a reciprocal attraction pervades the whole, modifying its nature and properties. Thus solution of sulphate of copper is blue, that of muriate of copper is green. Now, if into a solution of the former salt, we pour muriatic acid, we shall observe this robbing the sulphuric acid of a quantity of the cupreous oxide, proportional to its mass; for the more muriatic acid we add, the greener will the liquid become. But if, by concentration, the sulphate of copper be suffered to crystallize, the phenomenachange; a new force, that of crystallization, is superadded, which aids the affinity of the sulphuric acid, and decides the decomposition. The surplus of each of these acids is employed in counterbalancing the surplus of its antagonist, and need not be considered as combined with the copper. Here, however, we verge on the obscure and unproductive domain of chemical metaphysics, a region in which a late respectable systematist delighted to expatiate. M. Bethollet estimates the attractive forces or affinities of bodies of the same class, to be inversely as their saturating quantities. quantities. Thus, among acids, 50 parts of real sulphuric, will saturate as much potash or soda as 67+ of real nitric, and as 27 of carbonic. Thus too, 21 of ammonia will saturate as much acid as 25 of magnesia, 35t of lime, and 59 of potash. Hence he infers that the carbonic acid is endowed with a higher affinity than the sulphuric; and this, than the nitric. The same proposition applies to ammonia, magnesia, lime and potash. But in direct hostility to this doctrine, we have seen lime exercise a greater affinity for the acids than magnesia. And though M. Berthollet has ingeniously sought to explain away the difficulty about potash, ammonia, and carbonic acid, by referring to the solid or gaseous results of their action; yet it is hard to conceive of solidity operating in producing an effect, before solidity exists, and of elasticity operating while the substance is solid or liquid. On this point a good syllogism has been offered by Sir H. Davy. "The action," says this profound chemist, "between the constituents of a compound must be mutual. Sulphuric acid, there is every reason to believe, has as much attraction for barytes, as barytes has for sulphuric acid, and barytes is the alkaline substance of which the largest quantity is required to saturate sulphuric acid; therefore, on M. Berthollet's view, it has the weakest affinity for that acid; but less sulphuric acid saturates this substance than any other earthy or alkaline body. Therefore, according to M. Berthollet, sulphuric acid has a stronger affinity for barytes than for any other substance; which is contradictory." In the table of chemical equivalents at the end of the Dictionary, will be found a view of the definite proportions in which the various chemical bodies combine, referred to their primary or lowest numerical terms, vulgarly called the weights of the atoms. Mr. Higgins, the real author of the Atomic Theory, in first promulgating its principles in his Comparative View of the Phlogistic and Antiphlogistic Hypotheses, tions which would be requisite to ascer connected their exposition with general tain them. views of the relative forces of affinity among the combining particles. These forces he illustrates by diagrams, to which I have adverted, in the article EQUIVALENTS (CHEMICAL). This joint consideration of combining force and combining ratio, has been neglected by subsequent writers; whence, Mr. Higgins says, "The atomic doctrine has been applied by me in abtruse and difficult researches. Its application by Mr. Dalton, has been in a general and popular way; and it is from these eircumstances alone, that it gained the name of Dalton's Theory." Since the chemical statics appeared, perhaps no chemist has contributed so many important facts to the doctrines of affinity as M. Dulong. His admirable inquiries concerning the mutual decomposition of soluble and insoluble salts, were presented to the National Institute, and afterwards published in the Annales de Chimie, tom. 82; from which they were translated into the 35th and 36th volumes of Nicholson's Journal, and an abstract of them was given in the 41st vol. of the Phil. Mag. Notwithstanding such means of notoriety, it is amusing to observe so unwearied a compiler as Dr. Thomson, recently appropriating to his friend Mr. Phillips, the discovery of a fact observed and recorded years before by M. Dulong; and treating as an anomaly, what the French philosopher had shown to be none, but had referred with equal sagacity and industry to general principles. After the labours of Bergmann and Berthollet, chemistry seemed to leave little further to be desired, relative to the mutual decomposition of soluble salts. But the insoluble salts are likewise susceptible of exchanging their principles with a great number of the soluble salts. "This class of phenomena," says M. Dulong, "though almost as numerous as that which embraces the soluble salts, and capable of affording new resources to analysis, has not yet been examined in a general man ner." an op The action of the soluble carbonates on the insoluble salts, is the only one which had been at all studied. Thus carbonates of potash and soda in solution, had been employed conveniently to decompose sulphate of barytes. Μ. Dulong ulong had portunity in some particular researches, to observe a considerably extensive number of facts, relating to the mutual decomposition of the soluble and insoluble salts, and endeavoured, he says, to determine the general cause of these phenomena, and the method of foreseeing their results, without being obliged to retain by an effort of memory, of which few persons would be capable, all the direct observa M. Dulong found by experiment, that all the insoluble salts are decomposable by the carbonate of potash or the carbonate of soda, and in some instances with curious phenomena. When sulphate of barytes, phosphate of barytes, or oxalate of lime, is boiled with solution of bicarbo nate, or carbonate of potash, a considerable part of the insoluble sulphate is constantly transformed into a carbonate of the same base; but on reaching a certain limit, the decomposition stopped, although there remained sometimes a very considerable quantity of the soluble carbonate not decomposed. M. Dulong convinced himself, that the different degrees of concentration of the alkaline solution, produced but very slight variations in the results of this decomposition. He took 10 grammes of dry subcarbonate of potash, and 7.66, being their equivalent proportion, of dry subcarbonate of soda; quantities containing each 3.07 grammes of carbonic acid. They were separately dissolved in 250 grammes of water, and each solution was kept in ebullition for two hours, on 8 grammes of the sulphate of barytes. On analyzing the two residues, it was found that the potash experiment yielded 2.185 grammes, and the soda only 1.833; or in the proportion of 6 to 5. Is this difference to be ascribed to the difference in the attractive forces of the two alkalis; to the more sparing solubility, or greater attractive force of the sulphate of potash; or to both causes conjointly? Since the alkaline carbonates lose their decomposing agency when a certain proportion of the alkaline sulphate is formed, M. Dulong tried to ascertain the limits by the following experiment: 7 grammes of sulphate of potash, with 6 of subcarbonate, dissolved in 250 of water, were boiled with the sulphate of barytes for several hours, without the least trace of decomposition being evinced. The supernatant liquid, filtered, and boiled on carbonate of barytes, produced a considerable quantity of sulphate; but ceased acting before this sulphate of potash was exhausted. The same phenomena were obtained with carbonate and sulphate of soda. "Lastly, the sulphate of potash and the sulphate of soda alone, and perfectly neutral, re-acted likewise upon the carbonate of barytes, and produced on one part, sulphate of barytes, but on the other the subcarbonate of potash or soda which remained in solution, together with the portion of the sulphate which resisted the decomposition. 20 grammes of crystallized sulphate of soda, and 10 grammes of sulphate of potash, were separately dissolved in 260 of water, Each solution was boiled for 2 hours on 20 grains of carbonate of barytes. The sulphate of soda produced 10.17 gr. of sulphate of barytes, and the sulphate of potash 9.87." Had 108 of sulphate of potash been employed, which is the true equivalent of 200 sulphate of soda in crystals, a somewhat larger product would have been obtained than 9.87. This experiment, however, is most satisfactory with regard to the amount of decomposition. The mutual action of the insoluble carbonates, with the soluble salts, whose acids form, with the bases of these carbonates, insoluble salts, is equally general with that of the soluble carbonates on the insoluble salts. The following is M. Dulong's table of results: All those salts which have ammonia for their base, are completely decomposed by the insoluble carbonates found in the same column. The new insoluble salt replaces the carbonate which is decomposed, and the carbonate of ammonia flies off. Hence, if a sufficient quantity of insoluble carbonate be present, the liquid will become pure water. When the soluble salt has an insoluble base, the decomposition does not meet with any obstacle, but continues until the liquid becomes mere water. Thus, solution of sulphate of magnesia, boiled with carbonate of barytes, will be resolved into an insoluble carbonate and sulphate, provided enough of carbonate of barytes be present. Otherwise a portion of the magnesian carbonate being dissolved in its own sulphate, gives alkaline properties to the solution. If the base be metallic, it almost always forms a salt with excess of oxide, which being insoluble, precipitates. The general inferences of M. Dulong's inquiries are the following: 1. That all the insoluble salts are decomposed by the subcarbonates of potash or soda, but that a mutual exchange of the principles of these salts cannot in any case be completely made; or in other words, that the decomposition of the subcarbonates is only partial. 2. That all the soluble salts, of which the acid forms, with the base of the insoluble carbonates, an insoluble salt, are decomposed by these carbonates, until the decomposition has reached a certain limit which it cannot pass. When a soluble subcarbonate acts on an insoluble salt, in proportion as the carbonic acid is precipitated on the base of the insoluble salt, it is replaced in the solution by a quantity of another acid, capable of completely neutralizing the alkali. Thus, during the whole course of the decomposition, fresh quantities of neutral salt replace the corresponding quantities of an imperfectly saturated alkaline compound; and if we view the excess of alkaline pow er in the undecomposed subcarbonate, or its unbalanced capacity of saturation, as acting upon both acids, it is evident that in proportion as the decomposition advances, the liquid approaches more and more to the neutral state. In the inverse experiment, a contrary change supervenes. Each portion of the acid of the soluble salt, (sulphate of soda for example), which is precipitated on the base of the insoluble carbonate, is replaced by a quantity of carbonic acid, which forms with the corresponding base, an alkaline subcarbonate; and the more of the first acid is precipitated upon the earthy base, the more subcarbonate the liquid contains, and the further does its state recede from neutralization. This consideration seems to lead directly to the following theory of these decompositions. It is known, says M. Dulong, that all the salts, even those which possess the greatest cohesion, yield to caustic potash or soda, a more or less considerable portion of their acid, according to circumstances. Now the alkaline subcarbonates may be considered as weak alkalis, which may take from all the insoluble salts a small quantity of their acids. This effect would soon be limited if the alkali were pure, in consequence of the resistance offered by the pure and soluble base. But the latter meeting in the liquid, an acid with which it can form an insoluble salt, unites with it, and thus re-establishes the primitive conditions of the experiment. The same effects are produced successively on new portions of the bodies, till the degree of saturation of the liquid is in equilibrium with the cohesive force of the insoluble salt, so that the feebler this resistance may be, the more progress the decomposition will make. And again, when an insoluble carbonate is in contact with a neutral soluble salt, the base of the carbonate will tend to take part of the acid of the neutral salt; and if, from this union, an insoluble salt can result, the force of cohesion peculiar to this compound, will determine the forma tion. The carbonic acid, released from the attraction of the earthy base by the fixed acid, instantly attaches itself to the surrounding alkali, forming a subcarbonate which replaces the decomposed neutral salt. The precipitation of the fixed acid on the insoluble carbonate, and the absorption of carbonic acid by the liquid continues, until the alkalinity thereby developed, becomes so strong as to resist the precipitation of the acid; thus forming a counterpoise to the force by which that precipitation was accomplished. All action then ceases, so that the more cohesion the insoluble salt possesses, the greater will be the proportion of acid taken from the soluble salt. When the carbonate of potash can no longer decompose the sulphate of barytes, the carbonic acid which remains in the solution, is to the sulphuric acid nearly in the ratio of 3 to 1; and when the sulphate of potash can no longer act upon the carbonate of barytes, these two acids are nearly in the ratio of 3 to 2; whence it follows, that the first liquor is much more alkaline than the second. It is easy to account for this difference by examining the conditions of the equilibrium established in the two cases. When the sulphate of potash no longer decomposes the carbonate of barytes, it is because the excess of alkali, developed in the liquid, forms a counterpoise to the power with which sulphate of barytes tends to be produced in these circumstances. And when the subcarbonate of potash can no longer decompose the sulphate of barytes, it is because there is not such an excess of alkali in the liquid, as is capable of overcoming the cohesion and attraction between the elements of that salt. Now we know, that it requires a greater force to overcome an existing attractive power, than to maintain the quiescent condition. Therefore the subcarbonate of potash ought to cease to decompose the sulphate of barytes, before the sulphuric and carbonic acids are in the same relation in which they are found, when the equilibrium is established by the inverse experiment. Hence we see, that a mixture of sulphate and subcarbonate of potash, in which the proportions of their two acids shall be within the limits pointed out, will have no action either on the sulphate or carbonate of barytes. For the other insoluble salts, there will be other relations of quantity; but there is always a certain interval, more or less considerable, between their limits. The mutual action of sulphate of soda and carbonate of barytes is almost instantaneons. It is sufficient to pour a boiling hot solution of the sulphate, on the carbonate placed on a filter, in order that more than threefourths of the sulphuric acid be precipitated, and replaced by a corresponding quantity of carbonic acid. In the first part of the Philosophical Transactions for 1809, we have tables of elective attractions by Dr. Thomas Young, a philosopher of the very first rank, whom the late ingenious Dr. Wells pronounced the most learned man in England. These have been unaccountably overlooked by our different systematic writers, though they are, both in accuracy and ingenuity, far superior to the tables which, with unvarying routine of typography, are copied into their compilations, I conceive it will be doing an essential service to chemical students, to lay before them the tables of Dr. Young, accompanied with his admirable remarks on the sequences of double decompositions. Attempts have been made, by several chemists, to obtain a series of numbers, capable of representing the mutual attractive forces of the component parts of different salts ; but these attempts have hitherto been confined within narrow limits, and have indeed been so hastily abandoned, that some very important consequences, which necessarily follow from the general principle of a numerical representation, seem to have been entirely overlooked. It appears that nearly all the phenomena of the mutual actions of a hundred different salts may be correctly represented by a hundred numbers, while, in the usual manner of relating every case as a different experiment, above two thousand separate articles would be required. Having been engaged in the collection of a few of the principal facts relating to chemistry and pharmacy, Dr. Young was induced to attempt the investigation of a series of these numbers; and he has succeeded in obtaining such as appear to agree sufficiently well with all the cases of double decompositions which are fully established, the exceptions not exceeding twenty, out of about twelve hundred cases enumerated by Fourcroy. The same numbers agree in general with the order of simple elective attractions, as usually laid down by chemical authors; but it was of so much less importance to accommodate them to these, that he has not been very solicitous to avoid a few inconsistencies in this respect; especially as many of the bases of the calculation remain uncertain, and as the common tables of simple elective attractions are certainly imperfect, if they are considered as indicating the order of the independent attractive forces of the substances concerned. Although it cannot be expected that these numbers should be accurate measures of the forces which they represent, yet they may be supposed to be tolerable approximations to such measures; at least, if any two of them are nearly in the true proportion, it is probable that the rest cannot deviate very far from it: thus, if the attractive force of the phosphoric acid for potash is about eighttenths of that of the sulphuric acid for barytes, that of the phosphoric acid for barytes must be about nine-tenths as great. But they are calculated only to agree with a certain number of phenomena, and will probably require many alterations, as well as additions, when all other similar phenomena shall have been accurately investigated. "There must be a sequence," says Dr. Young, " in the simple elective attrac tions. For example, there must be an error in the common tables of elective at tractions, in which magnesia stands above ammonia under the sulphuric acid, and below it under the phosphoric; and the phosphoric acid stands above the sulphuric under magnesia, and below it under ammonia; since such an arrangement implies that the order of the attractive forces is this: phosphate of magnesia, sulphate of magnesia, sulphate of ammonia, phosphate of ammonia, and again phosphate of magnesia; which forms a circle, and not a sequence. We must therefore either place magnesia above ammonia under the phosphoric acid, or the phosphoric acid below the sulphuric under magnesia; or we must abandon the principle of a numerical representation in this particular case. "In the second place, there must be an agreement between the simple and double elective attractions. Thus, if the fluoric acid stands above the nitric under barita, and below it under lime, the fluate of barita cannot decompose the nitrate of lime, since the previous attractions of these two salts are respectively greater than the divellent attractions of the nitrate of barita and the fluate of lime. Probably, therefore, we ought to place the fluoric acid below the nitric under barita; and we may suppose, that when the fluoric acid has appeared to form a precipitate with the nitrate of barita, there has been some fallacy in the experiment. "The third proposition is somewhat less obvious, but perhaps of greater utility: there must be a continued sequence in the order of double elective attractions; that is, between any two acids we may place the different bases in such an order, that any two salts, resulting from their union, shall always decompose each other, unless each acid be united to the base nearest to it; for example, sulphuric acid, barita, potash, soda, ammonia, strontia, magnesia, glucina, alumina, zirconia, lime, phosphoric acid. The sulphate of potash decomposes the phosphate of barita, be cause the difference of the attractions of barita for the sulphuric and phosphoric acids is greater than the difference of the similar attractions of potash; and in the same manner, the difference of the attractions of potash is greater than that of the attractions of soda; consequently the difference of the attractions of barita must be much greater than that of the attractions of soda, and the sulphate of soda must decompose the phosphate of barita; and in the same manner it may be shown, that each base must preserve its relations of priority or posteriority to every other in the series. It is also obvious, that, for similar reasons, the acids may be arranged in a continued sequence between the different bases; and when all the decompositions of a certain number of salts have been investigated, we may form two corresponding tables, one of the sequences of the bases with the acids. and another of those of the acids with the different bases; and if either or both of the tables are imperfect, their deficiencies may often be supplied, and their errors corrected by a repeated comparison with each other." In the table of simple elective attractions, he has retained the usual order of the different substances; inserting again in parentheses such of them as require to be transposed, in order to avoid inconsequences in the simple attractions: He has attached to each combination marked with an asterisk the number deduced from the double decompositions, as expressive of its attractive force; and where the number is inconsistent with the corrected order of the simple elective attractions, he has enclosed it in a parenthesis. Such an apparent inconsistency may perhaps in some cases be unavoidable, as it is possible that the different proportions of the masses concerned in the operations of simple and compound decomposition may sometimes cause a real difference in the comparative magnitude of the attractive forces. Those numbers to which no asterisk is affixed, are merely inserted by interpolation, and they can only be so far employed for determining the mutual actions of the salts to which they belong, as the results which they indicate would fol low from the comparison of any other numbers, intermediate to the nearest of those which are more correctly determined. He was not able to obtain a sufficient number of facts relating to the me. tallic salts, to enable him to comprehend many of them in the tables. He thought it necessary to make some alterations in the orthography generally adopted by chemists, not from a want of deference to their individual authority, but because it appeared to him that there are certain rules of etymology, which no modern author has a right to set aside. |