Part 5

If to 24 parts of lime dissolved in water we put 98 parts of dry arsenic acid, the compound remains in solution, and is perfectly neutral to the colour test, but so that the addition of a small quantity of either ingredient disturbs the neutrality. If to this solution 24 parts of lime dissolved in water be added, the compound remains a limpid solution, but is very limy to the test. If to this we put in like manner, 24 parts more of lime, the whole compound is thrown down, and yields, when dried, 170 parts of arseniate of lime, the liquid being now free from both elements. Here we see first, two atoms of the deutoxide, neutralized by two atoms of base, namely, 1 of arsenic oxide, and 1 of lime; but (second), when one atom more of lime is added, an union of 2 deutoxide, and 3 of base is effected, which of course is an alkaline salt; when (third) more of lime is added, the 2 deutoxide and the 1 protoxide each attach 1 of lime, and form a still more alkaline salt, which being insoluble, is wholly thrown down, most probably in a compound state of 98 parts arsenic acid, combined with 72 parts lime.

In like manner, I find that 42 parts of potash, 28 of soda, and 12 of ammonia, severally neutralize 98 parts of arsenic acid.

1st. 24 lime + 32.7 arsenic acid = insoluble arseniate 2d. -- + 49 ------------ = soluble arseniate 3d. -- + 98 ------------ = neutral arseniate

It is a remarkable fact, that when neutral arseniate of potash and nitrate of lead are mixed together to mutual saturation, the precipitate is found to consist chiefly of arsenic acid and oxide of lead, in proportion of 1 of acid to two of oxide, (that is, 98 ∶ 194, or 100 ∶ 198); which does not differ much from the determination of Berzelius.

I find however, only one fourth of the nitric acid in the residuary liquid in a free state; which leads me to suspect that the precipitate is a compound of subnitrate and arseniate of lead, in which the arsenic acid and lead are in due proportion, or 98 acid, to 97 oxide. This consideration may be properly resumed hereafter.

Hence we conclude, the atom of arsenic weighs 21 (and not 42, as at page 264, VOL. 1), that of the protoxide of common white arsenic, 28; and that of arsenic acid = 98, being a compound of 2 atoms of deutoxide, and 1 of protoxide. Or,

100 Arsenic + 33.3 oxygen = 133.3 protoxide -- + 55.5 ---- = 155.5 arsenic acid

21. _Oxides of cobalt._

There are at least two oxides of cobalt, the one blue, the other black. Authors differ as to the proportions of the elements. Proust states the blue oxide to consist of 100 metal, and 19 or 20 oxygen, and the black of 25 or 26 oxygen. Klaproth finds in the blue, 100 metal and 18 oxygen. But Rolhoff according to Berzelius, finds 100 metal and 27.3 oxygen in the blue oxide, and 40.9 in the black. I have taken some pains to investigate these oxides, and have been able to satisfy myself in a good degree, respecting their constitution. The blue or protoxide consists of 100 metal and 19 oxygen, and the black oxide of 100 metal, and 25 or 26, very nearly as Proust determined.

_Protoxide._ By repeated trials I have found, that if 37 parts of metallic cobalt be treated with the due quantity of nitro-muriatic acid, and a heat of 150°, a rapid solution takes place, and a disengagement of pure nitrous gas; this being carefully collected, it will be found to weigh 8 grains, and of course corresponds to 7 grains of oxygen; hence 37 cobalt, unite to 7 oxygen, to form 44 of the blue oxide; and as this is the only oxide that combines with acids, it must be considered as the most simple or protoxide, being 1 atom of metal (37), and 1 of oxygen (7). The estimation of the atom of cobalt at 50 or 60, (page 265), must therefore be corrected.

_Compound oxides._ When the blue oxide of cobalt is precipitated from a solution, by an alkali or lime water, and oxymuriate of lime is gradually dropped in, the precipitate changes colour rapidly; it passes from blue to green and olive, thence to a dark bottle green, and finally becomes black; oxygen gas is given out copiously when an excess of oxymuriate of lime is used. I find the additional oxygen requisite to convert the blue to the black oxide is what Proust states it, namely, ⅓ of that necessary to form the blue; hence it must be considered as a compound of 1 atom of oxygen and 3 of the protoxide. Probably the other coloured oxides are 1 to 4, 1 to 5, &c. The protoxide is blue when precipitated, but it is supposed to contain water, or to be a hydrate; as it is dark grey when heated. The blue oxide in a short time after precipitation being still under water, changes to a yellowish or dead-leaf colour; which also appears to be a hydrate of the protoxide, as it dissolves in acids without giving out gas, and yields the blue oxide by an alkali. According to Proust, this hydrate contains 20 or 21 per cent. water. If we suppose the blue to be 1 atom oxide, and 1 water, the yellow hydrate may be 1 water and 2 of the proto-hydrate; or 88 oxide, and 24 water, which will be nearly 21 per cent. water.

The black oxide gives out oxygen gas by a red heat, and is reduced to the grey oxide: it forms oxymuriatic acid, with muriatic acid, and the protoxide remains in solution.

(See Tassaert.--An. de Chimie 28; Thenard, 42; and Proust, 60.)

22. _Oxides of manganese._

One of the oxides of manganese being a natural production, and sometimes of great purity, and the metal not being obtainable without skill and labour, it may be most convenient to adopt the inverse method in our investigations; that is, to trace out the atom of metal from its oxides.

_Native oxides of manganese._ Of late, I have met with excellent specimens of this oxide; they are in masses of a grey, crystalline appearance, sp. gr. 4, easily pulverizable into a greasy, shining, dark grey powder. They are nearly pure oxide; but the more common sort is blacker, and contains less or more of siliceous earth. Some specimens are very harsh, require an iron mortar to pulverize them, and contain 50 or upwards per cent. of siliceous earth. Of the common sort when pulverized, the black inclining to blue, is generally preferable to the black inclining to brown. I have not observed any earthy carbonates mixed with the oxide of manganese. Amongst various specimens I obtained the following analyses.

Oxide. sand and insoluble matter. 1. Grey, crystallized oxide 100 ---- 2. Pulverized black oxide, from } 80 20 a bleacher, reputed good } 3. Another specimen, in the lump 77 23 4. A light brown oxide 47 53 5. A sparry oxide, abounding with } 27 73 flint; black brown when pulverized }

Some of the chemical characters of the native oxide of manganese are, its giving oxygen gas by a red heat, its insolubility in nitric and sulphuric acids, and its solubility in muriatic acid, but with the accompanying circumstance of disengaging oxymuriatic acid.

All these facts shew that it is of the higher order of oxides, or analogous to the brown and red oxides of lead.--The muriatic acid solution abovementioned, contains an oxide of an inferior degree, which is soluble in all acids, and which is the only oxide of manganese that appears to be soluble in acids. If this be considered, (as it may with the greatest probability), the protoxide, then it will appear from what follows, that the common native manganese is the deutoxide, and that there is an intermediate one, which contains a mean quantity of oxygen.

_Protoxide._ This may be obtained in solution with muriatic acid as above, from the native oxide. Or the black oxide may be mixed with sulphuric acid into a paste, and heated in an iron spoon to redness; the mass being lixiviated, a solution of the protoxide in sulphuric acid is obtained, generally with a slight excess of the acid; in this process heat and the presence of sulphuric acid, expels the redundant oxygen of the black oxide, and reduce it to the protoxide, which hence becomes soluble. If in either of these solutions any oxide of iron be present, whether from the manganese, or acquired during the manipulation, it is easily discovered and separated, as I have frequently found. Into any solution containing a mixture of the oxides of manganese, the green oxide of iron, and the red oxide of iron, let lime water be gradually poured; the red oxide of iron will be first precipitated, next the green oxide, and lastly the oxide of manganese, which may hence be separated from each other. Iron may also be discovered and separated by carbonate of potash, which must be dropped into the solution as long as any coloured precipitate appears; as soon as it has subsided, the snow-white carbonate of manganese succeeds. This white carbonate may be very conveniently used for obtaining solutions of pure manganese in any of the acids.

When a solution of pure manganese is treated with lime water, or ammonia, a light buff oxide, not much differing in appearance from the yellow oxide of iron, is obtained. This oxide is soluble in all acids, when recently precipitated; but, such is its avidity for oxygen, with moderate agitation of the liquid it acquires oxygen and becomes brown, when it ceases to be totally soluble; if dried in the air quickly, it becomes brown and obtains considerable oxygen. The buff oxide recently precipitated, is probably a hydrate; for, when the white carbonate of manganese is heated gradually to red, the water and the acid are both expelled, and a grey powder remains; this is quite black on the surface of the mass, if exposed to the air during the process. Probably this grey powder is the pure protoxide; it is soluble in acids, except the black powder at the surface; perhaps but for the oxygen of the air, the protoxide would be nearly white.

From its combinations with sulphuric and carbonic acids, I find the weight of an atom of the protoxide to be 32, or the same as that of iron. Dr. John, a German chemist, who seems to have investigated these salts with more attention than any other person, has deduced nearly the same results. (Annals of Philos. 2-172). He finds 33⅔ sulphuric acid + 31 oxide, and 34.2 carbonic acid + 55.8 oxide; that is, when reduced to compare with my results, 34 sulphuric acid + 31.3 oxide, and 19.4 carbonic acid + 32 oxide. This near agreement may be considered as a confirmation of the accuracy of both. Dr. John finds, as I have done, three distinct oxides of manganese, the greyish green, the brown, and the black. The first of these is the only one that combines with acids; but we differ materially as to the quantity of oxygen in each. He found manganese decompose water at the ordinary temperature; by oxidizing the metal this way, 100 metal acquired 15 oxygen to constitute the protoxide; according to this, 28 metal + 4 oxygen would make 32 protoxide; but this conclusion would be so contrary to all analogy, that it cannot be admitted as satisfactory. The probability is, that the manganese must have contained a little oxygen at the commencement of the experiment. The general analogy of manganese to iron, lead, &c. requires that 32 protoxide should contain 7 oxygen. If this be allowed, we have the atom of manganese = 25, and not 40, (as at page 266, VOL. 1), the same as that of iron: and this conclusion is corroborated by what follows.

2. _Intermediate or olive brown oxide._ This may be formed by combining oxygen directly with the buff or protoxide recently precipitated, and still remaining in the liquor; simple agitation in oxygenous gas or common air for a few minutes, is all that is requisite. Or it may be instantly formed by treating the same moist protoxide with liquid oxymuriate of lime. Or it may be had by exposing the purest black oxide to a bright red heat for some time, when it will lose 9 or 10 per cent. and there will remain the olive brown oxide.

To find the proportion of oxygen absorbed, I precipitated 3.2 grains of the protoxide by lime water; the liquid containing the oxide was put into a well stoppered bottle of oxygen gas; on agitation the oxide changed colour fast, from buff to brown; in a short time it absorbed 260 grain measures of gas = .35 of a grain in weight, and then ceased to absorb. In another experiment, 3.2 grains of precipitated protoxide, took 100 measures of a solution of oxymuriate of lime, containing .35 per cent. of oxygen, (that is, 1.45 oxymuriatic acid). Hence as 32 take 3.5, 64 must take 7; which shews the brown oxide to be a compound of 1 atom of oxygen, and 2 of the protoxide.

The characters of this oxide are, its olive brown colour, its insolubility in nitric and sulphuric acids, without heat or deoxidation, and its solubility in muriatic acid after the evolution of oxymuriatic acid. By long exposure to the air, it is gradually changed, in all probability into the black oxide.

3. _Deutoxide._ In order to determine the quantity of oxygen deducible from the purest native oxide of manganese, to convert it into protoxide, I have successfully adopted the two following methods. 1st. Let 39 or 40 grains of the oxide be mixed with 60 common salt; to this add 80 grains of water, and 120 grains weight of strong sulphuric acid, in a gas bottle. The heat must be gradually raised to boiling, and the oxymuriatic acid gas may be received in a quart of lime water. This will be found sufficient to convert 800 measures of test green sulphate of iron (1.156) into red; that is, it will produce 29 grains of oxymuriatic acid, which will cause 7 grains of oxygen, to unite to the green oxide of iron. Now 100 measures of 1.156 sulphate, according to some recent experiments of mine, contain 8 grains of green oxide, (I estimated the sp. gr. of test sulphate, heretofore at 1.149); hence 800 contain 64 oxide, and these require just 7 grains of oxygen to be united to them, to form the red oxide, as has been shewn, page 34. In the above experiment, the 39 grains of oxide, will be found to vanish or be dissolved, if pure, and to yield 32 grains of protoxide, making up with the 7 grains of oxygen, the original weight. Hence we have 39 grains of the oxide resolved into 32 protoxide, and 7 oxygen. If then we allow 32 protoxide, to contain 7 oxygen, it appears that 39 grains of the native oxide, consists of 1 atom manganese (25), and two atoms of oxygen (14); or it is the deutoxide of the metal. 2d. A more direct and expeditious method, of transferring the oxygen from the manganese to the iron, is as follows: Let 39 grains of pure grey shining oxide, be mixed with 800 of test green sulphate of iron; to this mixture let 25 or 30 grain measures of strong sulphuric acid be added: after stirring the mixture for 5 minutes, the oxide of manganese will be completely dissolved, and, on precipitating the oxide of iron gradually, by lime water, it will be found to be wholly _yellow_ or buff; shewing that 7 grains of oxygen have been transferred from the oxide of manganese to that of iron.--If more green sulphate of iron be used, then the surplus of the oxide will be thrown down green; the order of precipitation being the yellow oxide of iron, the green oxide of iron, and lastly, the yellow or buff oxide of manganese, as has been stated. This affords an easy and elegant method of appreciating the different oxides of manganese of commerce; and it was in this mode, the valuations of the specimens in the above table were made.

The proportions of the three oxides are then as under:

Manganese Oxygen Protoxide 100 + 28 -- buff; soluble in acids. Intermediate oxide -- + 42 -- brown; insoluble. Deutoxide -- + 56 -- black; insoluble.

It may be proper to subjoin the results of others, who have investigated the oxides of manganese. Bergman finds 3 oxides, containing 100 metal + 25, 35, and 66.6 oxygen; Dr. John finds 3 oxides, containing 100 metal + 15, 25, and 40 oxygen: Berzelius finds 5 oxides, containing 100 metal + 7, 14, 28, 42, and 56 oxygen; and Davy finds 2 oxides, containing 100 metal + 26.6, and 39.9 oxygen, respectively.

23. _Oxides of chromium._

There appear to be at least two oxides of chromium, one or other of which is found in combination with the oxides of lead or iron, but hitherto so very sparingly that few chemists have had an opportunity of investigating the proportions of chrome and oxygen, in the oxides of chromium. The chief sources for information on this subject, are essays by Vauquelin, An. de Chimie, VOL. 25 and 70; by Tassaert, _ibid._ 31; by Mussin Puschin, _ibid._ 32; by Godon, _ibid._ 53; by Laugier _ibid._ 78, and by Berzelius, Annal. of Philosophy, 3.

The oxides of chromium, as might be supposed, are distinguished for the colours which they possess and impart to the compounds into which they enter. One of the oxides is green; it gives colour to the emerald. The other is yellow, dissolved in water, but deep red when crystallized, and possesses the characters of an acid; it unites with alkalies, earths, and metallic oxides; it was first found in Siberia, in combination with the oxide of lead, a salt now denominated _chromate_ of lead, of a splendid yellow colour, inclining to orange or red. Since then, the chromate of iron, has been found in France, America, and Siberia, with a prospect of greater abundance.

In order to investigate the weight of the atom of chromic acid, it is necessary to attend to such of the chromates as have been carefully examined. The chromates of potash, barytes, lead, iron, and mercury, are those with which we are best acquainted.

Vauquelin has given us the components of the native chromate of lead by analysis, and those of the artificial chromate by synthesis; the results do not accord very nearly: for, according to the analysis corrected by the modern science,

Chromate of lead = 62 acid + 97 oxide By synth. chromate of lead = 57½ -- + 97 --

Berzelius however, has more lately given us the results of his experience, both analytical, and synthetical; and he finds both to give chromate of lead nearly = 44 acid + 97 oxide.

Chromate of barytes (Vauq.) = 47.8 acid + 68 barytes Ditto (Berz.) = 44 -- + 68 -- Native chromate of iron (Vauq.) = 45 acid + 35½ oxide Ditto (Laugier) = 55 -- + 35½ --

Having received a small portion of chromate of potash in solution, from a chemical friend (J. Sims), I endeavoured to satisfy myself, as far as my materials would go, as to the nature and proportions of the chromates. The solution was of the sp. gr. 1.061, and consequently in 100 measures contained nearly 6.7 grains of chromic acid and potash, &c.--The liquid was a beautiful yellow; it was alkaline by the colour test. By the usual tests, I had reason to believe, that the solution contained as under per cent.--namely,

2.2 gr. chromic acid 2. potash .8 uncomb. potash 1.4 carb. potash .3 sulphate of potash --- 6.7

With this liquid neutralized by nitric acid, I formed the chromates of lead, barytes, iron, and mercury; and I am inclined to believe these salts are nearly constituted as under:

Neutral chromate of potash 46 acid + 42 potash -- -- of barytes 46 -- + 68 barytes -- -- of lead 46 -- + 97 oxide -- -- of iron 46 -- + 32 oxide (black) -- -- of mercury 46 -- + 174 oxide (black)

According to these results, the atom of chromic acid weighs 46; it is made 44 by the results of Berzelius, and from 45 to 62 by those of Vauquelin; I would not be understood to place great confidence in the above results of mine, though I am persuaded they will be found good approximations.

Is the chromic acid the deutoxide, or the tritoxide of chromium?

The determination will evidently be affected by the question, how much oxygen must be abstracted from the chromic acid to reduce it to the green oxide. Vauquelin finds 46 acid to lose 6½ oxygen, and Berzelius 10½, when converted into green oxide by heat. From the former of these, one would infer chrome to be 32, the green or protoxide of chrome to be 39, and the acid or deutoxide 46: from the latter, chrome = 25, protoxide = 32 (unknown), the green oxide = 1 protoxide and 1 deutoxide united [= 71 = 50 chrome + 21 oxygen = (25 chrome + 10½ oxygen) × 2 = 35½ × 2] the deutoxide = 39, and the tritoxide or chromic acid = 46. I have not had an opportunity to perform any experiment that appears to me decisive as to the accuracy of one or other of these views; but shall make a few remarks relative to them.