ALUM.
At page 531, vol. 1, we have given the constitution of this important salt, as under: since that time Mr. R. Phillips has announced another view of it; and Dr. Thomson has published one differing from both of these. They are as follow:
| Dalton— | 1 atom sulphate of potash. |
| 4 atoms sulphate of alumine. | |
| 30 atoms water. | |
| Phillips— | 1 atom bisulphate of potash. |
| 2 atoms sulphate of alumine. | |
| 22 atoms water. | |
| Thomson— | 1 atom sulphate of potash. |
| 3 atoms sulphate of alumine. | |
| 25 atoms water. | |
Notwithstanding these differences, there is a near approximation in all three, in regard to the quantities of acid, alumine, potash, and water in the salt. This is accounted for partly in the different relative weights of the atoms, as estimated by the different analysts, but chiefly in that of alumine.
Some very curious results occurred to me about 10 years ago in analysing alum; they were new to me, but I have since found they had been previously discovered by Scheele. (See his essay on silex, clay, and alum, 1776.) As his observations are not to be found in any of our elementary books that I have seen, I shall give the particulars of my own experiments here.
I take 24 grains of alum and dissolve them in water; of these 8 grains may be allowed for sulphuric acid, ⅕ of which = 1.6 grain = 1.1 grain of lime = 880 grains of lime water, such as I commonly use. To the solution of alum I put 880 grains of lime water; a slight precipitate appears which soon becomes redissolved almost completely. The liquid is then acid by the colour test.
To this liquid I put 880 more of lime water, and agitate; a copious precipitate appears and continues; after subsidence the clear liquid is still acid by the colour test.
Another 880 grains are added, and the whole is then well agitated; the agitation is repeated two or three times after the precipitate has partly subsided, so as to diffuse it equally again through the liquid; finally, the clear liquid is found to be neutral by the colour test, and to contain no alumine; for, lime water produces no precipitate when poured into it.
Another 880 grains being added, and the whole stirred well, the clear liquid after the subsidence of the precipitate is still neutral by the colour test.
The fifth portion of 880 grains being then added, and the mixture well agitated, a considerable portion of the precipitate will evidently disappear, and the mixture become semitransparent; after a time the clear supernatant liquid is found strongly alkaline; a little of it touched with an acid becomes milky, and adding more acid clears it again. The liquid is now 1.0025 sp. gr., or a little heavier than lime water.
The sixth portion of 880 grains being now added to the whole mixture, and agitated, the precipitate rather diminishes, and an increase of specific gravity takes place in the liquid; it is now 1.003.
The seventh and last portion of 880 grains being added to the mixture, and agitation being continued for some time, a dense bulky precipitate is formed, which falls with great celerity, carrying with it the greatest part of the acid, the alumine and the lime, and leaving the liquid of the sp. gr. 1.0012. It is a subsulphate into which acid, potash, lime and alumine enter, as will be shewn.
These phenomena appear to me to be best explained by adopting a constitution of alum, such as to make it consist of 1 atom bisulphate of potash and 3 atoms of sulphate of alumine; after which the following explanation will apply.
The first portion of lime water saturates the excess of acid.
The second portion throws down a correspondent portion of alumine. The clear liquid is acid, because it contains sulphate of alumine, which is essentially acid by the colour test, because alumine is not an alkaline element.
The third portion throws down another portion or atom of alumine; but by continued agitation the two atoms of alumine liberated, join the remaining atom of sulphate of alumine, and the whole compound falls down, being then the common subsulphate of alum. Hence the liquid, containing nothing but sulphate of lime and sulphate of potash, is neutral by the test, and yields no alumine by the addition of lime water.
The fourth portion of lime water being put in and duly agitated, the atom of sulphuric acid is drawn from the subsulphate to join the lime, and then the floating subsulphate of alumine becomes pure alumine, and the clear liquor is still neutral.
The fifth portion of lime water tries to decompose the sulphate of potash, but is unable of itself; however, the floating alumine assists it, and by double affinity the potash leaves the acid to join the alumine, and the lime takes the acid. Hence as ⅓ of the alumine enters into solution with the potash, the precipitate is less copious, and the liquid is alkaline; a small portion of acid put into the clear liquid engages the potash, and liberates the alumine, but a larger portion redissolves the alumine also.
The sixth portion of lime water seems to complete the effect which the fifth commences, and hence the density of the liquid increases, whilst the precipitate rather diminishes.
The seventh portion of lime, together with the sixth, after due agitation and some time, unite the lime with the alumine, one atom of each, and form a precipitate which would fall together, were no other compound present, as I found, and Scheele before me; but if sulphate of lime be present, each compound atom of lime and alumine, unites with one of sulphate of lime, and the whole descends together, forming a subsulphate resembling that of alum, only two atoms of lime are found as substitutes for two atoms of alumine. This subsalt is very little soluble in water.
According to this view, if 2 atoms of alum were decomposed, 4 atoms of subsulphate would be formed, each consisting of 1 acid, 2 lime, and 1 alumine; also 2 compound atoms of potash and alumine, and 6 atoms sulphate of lime. But in the final arrangement, it would seem, that 2 atoms of sulphate of lime are again decomposed, and sulphate of potash formed, the 2 atoms of lime combining with the 2 of alumine, and then two more atoms of subsulphate are formed, and the final arrangement is 6 atoms subsulphate precipitated, and 2 atoms sulphate of potash, and 2 sulphate of lime remain in solution.
The facts above stated appear to me to place the constitution of alum in a clearer point of view than any other I have seen. They make no difference in the weights of the several elements in 100 grains of the salt, from what we have given in Vol. 1; only the weight of the atom of alumine is here taken to be 20 instead of 15, and we have 3 atoms of it in 1 of alum, instead of 4, as in the former account.
ON THE PRINCIPLES OF THE ATOMIC SYSTEM
OF CHEMISTRY.
It is generally allowed that the great objects of the atomic system are, 1st to determine the relative weights of the simple elements; and 2d to determine the number, and consequently the weight, of simple elements that enter into combination to form compound elements. The greatest desideratum at the present time is the exact relative weight of the element hydrogen. The small weight of 100 cubic inches of hydrogen gas, the important modifications of that weight by even very minute quantities of common air and aqueous vapour, and the difficulties in ascertaining the proportions of air and vapour in regard to hydrogen, are circumstances sufficient to make one distrust results obtained by the most expert and scientific operator. The specific gravity of hydrogen gas was formerly estimated at ⅒ that of common air; it descended to ¹/₁₂.₅, which is the ratio we adopted in the Table at the end of Vol. 1. it is now commonly taken to be ¹/₁₄.₅, and whether it may not in the sequel be found to be ¹/₁₆.₅ is more than any one at present, I believe, has sufficient data to determine. The other factitious gases have mostly undergone some material alterations in their specific gravities in the last twenty years, several of which I have no doubt are improvements; but when we see these specific gravities extended to the 3rd, 4th, and 5th places of decimals, it appears to me to require a credit far greater than any one of us is entitled to. In the mean time, it may be thought a fortunate circumstance, that the weight of common air has undergone no change for the last thirty or forty years; 100 cubic inches bring estimated to weigh 30.5 grains at the temperature of 60°, and pressure of 30 inches of mercury: (whether this is exclusive of the moisture I do not recollect.) It is also a fortunate circumstance, (provided it be correct) that this weight is nearly free from decimal figures. I may be allowed to add, that according to my experience, the weight of 100 cubic inches of air is more nearly 31 grains than 30.5. I apprehend these observations are sufficient to shew that something more remains to be done before we obtain a tolerably correct table of the specific gravities of gases; the importance of this object can not be too highly estimated.
The combinations of gases in equal volumes, and in multiple volumes, is naturally connected with this subject. The cases of this kind, or at least approximations to them, frequently occur; but no principle has yet been suggested to account for the phenomena; till that is done I think we ought to investigate the facts with great care, and not suffer ourselves to be led to adopt these analogies till some reason can be discovered for them.
The 2d object of the atomic theory, namely that of investigating the number of atoms in the respective compounds, appears to me to have been little understood, even by some who have undertaken to expound the principles of the theory.
When two bodies, A and B, combine in multiple proportions; for instance, 10 parts of A combine with 7 of B, to form one compound, and with 14 to form another, we are directed by some authors to take the smallest combining proportion of one body as representative of the elementary particle or atom of that body. Now it must be obvious to any one of common reflection, that such a rule will be more frequently wrong than right. For, by the above rule, we must consider the first of the combinations as containing 1 atom of B, and the second as containing 2 atoms of B, with 1 atom or more of A; whereas it is equally probable by the same rule, that the compounds may be 2 atoms of A to 1 of B, and 1 atom of A to 1 of B respectively; for, the proportions being 10 A to 7 B, (or, which is the same ratio, 20 A to 14 B,) and 10 A to 14 B; it is clear by the rule, that when the numbers are thus stated, we must consider the former combination as composed of 2 atoms of A, and the latter of 1 atom of A, united to 1 or more of B. Thus there would be an equal chance for right or wrong. But it is possible that 10 of A, and 7 of B, may correspond to 1 atom A, and 2 atoms B; and then 10 of A, and 14 of B, must represent 1 atom A, and 4 atoms B. Thus it appears the rule will be more frequently wrong than right.
It is necessary not only to consider the combinations of A with B, but also those of A with C, D, E, &c.; as well as those of B with C, D, &c., before we can have good reason to be satisfied with our determinations as to the number of atoms which enter into the various compounds. Elements formed of azote and oxygen appear to contain portions of oxygen, as the numbers 1, 2, 3, 4, 5, successively, so as to make it highly improbable that the combinations can be effected in any other than one of two ways. But in deciding which of those two we ought to adopt, we have to examine not only the compositions and decompositions of the several compounds, of these two elements, but also compounds which each of them forms with other bodies. I have spent much time and labour upon these compounds, and upon others of the primary elements carbone, hydrogen, oxygen, and azote, which appear to me to be of the greatest importance in the atomic system; but it will be seen that I am not satisfied on this head, either by my own labour or that of others, chiefly through the want of an accurate knowledge of combining proportions.
NEW TABLE
OF THE RELATIVE WEIGHTS OF ATOMS.
At the close of the last volume, the weights of several principal chemical elements or atoms were given; but as several additions and alterations have been educed from subsequent experience, it has been judged expedient to present a reformed table of weights.
SIMPLE ELEMENTS.
| Weights. | |
| Hydrogen | 1 |
| Azote | 5±, or 10? |
| Carbone | 5.4 |
| Oxygen | 7 |
| Phosphorus | 9 |
| Sulphur | 13, or 14 |
| Calcium | 17? |
| Sodium | 21 |
| Arsenic | 21 |
| Molybdenum | 21, or 42? |
| Cerium | 22? |
| Iron | 25 |
| Manganese | 25 |
| Nickel | 26 |
| Zinc | 29 |
| Tellurium | 29, or 58? |
| Chromium | 32 |
| Potassium | 35 |
| Cobalt | 37 |
| Strontium | 39 |
| Antimony | 40 |
| Iridium | 42 |
| Palladium | 50 |
| Uranium | 50, or 100? |
| Tin | 52 |
| Copper | 56, or 28? |
| Rhodium | 56 |
| Titanium | 59? |
| Gold | 60± |
| Barium | 61 |
| Bismuth | 62 |
| Platina | 73 |
| Tungsten | 84, or 42? |
| Silver | 90 |
| Lead | 90 |
| Columbium | 107? 121? |
| Mercury | 167, or 84? |
SIMPLE OR COMPOUND?
| Weights. | |
| Fluoric Acid | 10? 15? |
| Magnesia | 17 |
| Alumine | 20 |
| Glucine | 23? 34? |
| Lime | 24 |
| Oxymuriatic Acid (chlorine) | 29, or 30 |
| Muriatic Acid | 30, or 31 |
| Gas | |
| Zircone | 45 |
| Silex | 45? |
| Yttria | 53? 36? 18? |
COMPOUND ELEMENTS.
| Weights. | |
| Ammonia | 6? 12? 13? |
| Olefiant Gas | 6.4? 12.8? |
| Carburetted Hydrogen | 7.4 |
| or Pond Gas | |
| Water | 8 |
| Phosphuretted Hydrogen | 10 |
| Nitrous Gas | 12, or 24? |
| Carbonic Oxide | 12.4 |
| Sulphuretted Hydrogen | 15 |
| Deutoxide of Hydrogen | 15 |
| Nitrous Oxide | 17 |
| Nitrous Acid | 19, or 38? |
| Carbonic Acid | 19.4 |
| Sulphurous Oxide | 21 |
| Phosphoric Acid | 23 |
| Sulphurous Acid | 28 |
| Protoxide of Arsenic | 28 |
| Soda | 28 |
| Hydrate of Lime | 32 |
| Protoxide of Iron | 32 |
| Protoxide of Manganese | 32 |
| Protoxide of Nickel | 33 |
| Sulphuric Acid | 35 |
| Sulphuret of Arsenic (native) | 35 |
| Hydrate of Soda | 36 |
| Oxide of Zinc | 36 |
| Carbonate of Magnesia | 36.4 |
| Protosulphuret of Iron | 39 |
| Deutoxide of Manganese | 39 |
| Oxide of Chromium | 39 |
| Muriate of Magnesia | 39 |
| Protosulphuret of Nickel | 40 |
| Protosulphuret of Lime | 41 |
| Carbonate of Lime | 43.4 |
| Protoxide of Cobalt | 44 |
| Strontites | 46 |
| Muriate of Lime | 46 |
| Chromic Acid | 46 |
| Protoxide of Antimony | 47 |
| Carbonate of Soda | 47.4 |
| Hydrate of Potash | 50 |
| Muriate of Soda | 50 |
| Sulphate of Magnesia | 52 |
| Sulphuret of Antimony | 54 |
| Sulphate of Alumine (simple) | 55 |
| Oxide of Palladium | 57 |
| Sulphate of Lime | 59 |
| Protoxide of Tin | 59 |
| Carbonate of Potash | 61.4 |
| Hydrosulphuret of Antimony | 62 |
| Nitrate of Magnesia | 62 |
| Sulphate of Soda | 63 |
| Protoxide of Copper | 63 |
| Muriate of Potash | 64 |
| Deutoxide of Tin | 66 |
| Protosulphuret of Tin | 66 |
| Oxide of Gold | 67 |
| Barytes | 68 |
| Muriate of Lime | 69 |
| Oxide of Bismuth | 69 |
| Deutoxide of Copper | 70 |
| Nitrate of Soda | 73 |
| Sulphuret of Gold | 74 |
| Protosulphuret of Bismuth | 76 |
| Sulphate of Potash | 77 |
| Oxide of Platina | 80? |
| Nitrate of Potash | 87 |
| Carbonate of Barytes | 87 |
| Muriate of Barytes | 90 |
| Oxide of Silver | 97 |
| Protoxide of Lead | 97 |
| Minium | 98 |
| Sulphate of Barytes | 103 |
| Deutoxide of Lead | 104 |
| Protosulphurets of Lead and Silver | 104 |
| Nitrate of Barytes | 113 |
| Protoxide of Mercury | 174? |
| Deutoxide of Mercury | 181? |
| Protosulphuret of Mercury | 181 |
| Alum | 277 |