Joseph Louis Gay-Lussac (1778-1850) and Louis Jacques Thenard (1777-1857)

"On the nature and the properties of muriatic acid and of oxygenated muriatic acid"

Mémoires de Physique et de Chimie, de la Société d'Arcueil, 2, 339-58 (1809) [translated and excerpted in Alembic Club Reprints #13 The Early History of Chlorine]


We had assured ourselves, while engaged on the decomposition of fluoric acid,[1] that muriatic gas was the only one of all the gases which contained combined water, and this singular exception required us to make new researches to determine whether this water was actually essential to its intimate constitution. We first of all sought to determine the quantity of water which could be extracted from muriatic gas, and we got at this in various ways. We dissolved 27.151 grams of muriatic gas in water, cooled to -20°, and precipitated by means of nitrate of silver. The muriate which resulted therefrom weighed 106.82 grams, representing 20.61 grams of dry acid, seeing that muriate of silver is composed of

100.00 silver
7.60 oxygen
25.71 acid,
consequently, the 27.151 grams of muriatic acid contained 6.54 grams of water, or 0.240 of its weight. This quantity of water contains very nearly enough oxygen to produce the oxide necessary for the saturation of the acid combined with it; for in muriate of silver, of which we have given the proportions, oxygen is to acid as 1:3.38; and in muriatic gas the ratio is that of 1 to 3.53.

In order to verify this result, we passed a current of muriatic gas over well-cleaned iron turnings at a dull red heat. Much hydrogen gas was disengaged, without sensible admixture of muriatic gas; much muriate of iron was at the same time obtained; the residual turnings were not oxidised, which proves that the muriatic gas contained exactly enough water to oxidise all the iron which it could dissolve. Consequently, from this result and the ratio of oxygen to silver in muriate of silver, muriatic acid must contain 0.251 of its weight of water.

Again, on decomposing oxygenated muriatic gas by ammoniacal gas, it is found to contain exactly the half of its volume of oxygen gas, and that consequently it can absorb an equal volume of hydrogen. It is further found that a mixture, in equal parts, of oxygenated muriatic gas and hydrogen gas changes, in the course of several days, into ordinary muriatic gas, and that no water is deposited. But as the specific gravity of oxygenated muriatic gas is, according to our experiments, 2.470, that of air being taken as unity, it follows that, on taking away the weight of a half part of oxygen, 0.5517, and adding to this the weight of one part of hydrogen, 0.0753, to form water, a quantity of muriatic acid represented by 2.470 + 0.0732 = 2.5432 contains 0.5517 + 0.0732 = 0.6249 of water, or 0.2457 of its weight. This quantity differs little from the two preceding, so that we may conclude that ordinary muriatic gas contains 0.25 of its weight of water.

These results show that muriatic gas contains enough water to oxidise the metals which it can dissolve. It would not be possible, or at least it would be very difficult to decompose it directly. It is for that reason that, without losing the hope of decomposing it, we sought to extract it from oxygenated muriatic gas, by removing the oxygen by means of combustible bodies, as we had found that oxygenated muriatic gas does not contain combined water.

It is not possible to make use of the metals, seeing that with oxygenated muriatic acid they form neutral muriates. It seemed that the sulphides should accomplish our purpose; but, on bringing them into contact with oxygenated muriatic gas, in place of muriatic acid we obtained the sulphur liquor discovered by Mr THOMSON. The sulphides of baryta and of lime did not give us any more satisfactory results; when they were slightly moistened the oxygenated muriatic acid was certainly decomposed; but much sulphurous acid gas was disengaged. When, on the contrary, they were perfectly dry, there was no action. Neither does phosphorus separate oxygen from oxygenated muriatic gas; they combine directly, and form the liquid which we discovered by distilling phosphorus with muriate of mercury.

Finally, as a last method, we tried to decompose oxygenated muriatic gas by charcoal ignited at the extreme heat of the forge. To avoid the presence of the smallest quantity of water we made the gas pass slowly through a large glass tube a meter and a half in length, filled with muriate of lime. This tube communicated with a porcelain tube in which the charcoal was exposed to a red heat. The first portions of the oxygenated muriatic gas were completely converted into ordinary muriatic gas. This effect diminished gradually in spite of a very great elevation of temperature, and soon the gas passed without alteration, mixed only, towards the end of the experiment, with one thirty-third of an inflammable gas, which we believe to be carbonic oxide gas. This result clearly showed us that oxygenated muriatic gas is not decomposed by charcoal, and that the muriatic gas which we had obtained at the commencement of the operation was due to the hydrogen of the charcoal, which had combined with the oxygen of the acid. In fact, on taking ordinary charcoal without igniting it, muriatic gas was disengaged during a lengthened period even at a temperature only slightly elevated, and this acid contained water, like that coming from the decomposition of muriate of soda by sulphuric acid. According as the charcoal lost its hydrogen, however, the quantity of muriatic acid went on diminishing, and finally nothing was obtained but oxygenated muriatic gas. Even plumbago changed the latter gas into ordinary muriatic gas, that is to say, muriatic acid containing water; and since the most strongly ignited charcoal produces the same change, we must conclude that it is the hydrogen gas contained in these bodies which is the true cause of the change. Oxygenated muriatic gas may thus be considered the most powerful agent that we know for depriving charcoal of its hydrogen, seeing that it demonstrates its presence even after the charcoal has been urged to the most violent heat of the forge.

After that we concluded that muriatic acid could not exist without water in the state of gas, and that oxygenated muriatic acid could not be decomposed except by bodies which contain hydrogen, or by those which, like the metals, sulphur, or phosphorus, can form triple compounds with it. New experiments confirmed us in this opinion. We found that dry oxygenated muriatic gas was not decomposed by sulphurous gas, nitrous oxide gas, carbonic oxide gas, or even by nitrous gas, provided that they also were perfectly dry; and that it was decomposed instantly by these same gases with the help of water. Only nitrous gas when mixed with oxygenated muriatic gas slightly altered its colour, changing it to a slightly orange green; but this we doubt not should be attributed to a little water or oxygen, as we had remarked that the drier and purer these two gases were, the less marked was the change of colour.

We know, on the authority of M. BERTHOLLET, that liquid oxygenated muriatic acid is decomposed by light, and, according to M. FOURCROY, that in the state of gas it is not decomposed by this agent, or even by a very high temperature. These facts manifestly prove that the affinity of water for muriatic acid co-operates in this decomposition, and they led us to try the decomposition of oxygenated muriatic gas by means of heat along with water. We therefore arranged an apparatus enabling us to introduce oxygenated muriatic acid gas, alone or mixed with the vapour of boiling water, into a porcelain tube exposed to a red heat. In the first case the gas did not suffer any alteration; but as soon as the vapour of water was introduced oxygen gas and muriatic acid were obtained. It is not necessary for this decomposition that the temperature should be very high, as it still takes place below a red heat.

The affinity of muriatic acid for water is such as to cause the decomposition of oxygenated muriatic gas by hydrogen gas at a temperature only a little higher than that of boiling water. If a mixture of equal parts of these two gases is made and a small piece of iron heated in mercury to 150° introduced, there is a violent inflammation, and formation of muriatic acid.

Here, then, we have a body, oxygenated muriatic gas, which is decomposed neither by light nor by heat, but which on the other hand, is decomposed quite easily by the one or the other with the help of water. On comparing the action of these two fluids, one cannot but admit that it is identical in every case when it is exercised upon unorganised bodies. M. RUMFORD drew the same conclusion from his experiments on the decomposition of solutions of gold and silver by charcoal, ether, and oils, by means of light and heat; but the decomposition of oxygenated muriatic gas, which it had not been possible to effect by heat, appeared then a very powerful objection. This exception had not escaped M. BERTHOLLET in his Statique Chimique, and he had added to it another which he had deduced from the difference in the action of heat and of light on nitric acid; he had, however, none the less considered their action as in general identical on almost all other bodies. Our experiments destroy these two objections, and no doubt can remain that light acts on unorganised bodies in the same manner as heat. It is enough even, in order to form a conception of the effects of the former, to admit with M. RUMFORD that it raises the temperature of the molecules on which it acts, while that of the substance in which these molecules may happen to be receives only a slight augmentation.

The decomposition of oxygenated muriatic gas by light is progressive, as it depends on the intensity of the latter. This observation suggested to us that, in a great number of cases in which compounds were formed only slowly, this arose from the fact that they were produced by an agent such as light which, in a very short time, could produce only very small effects, being in small quantity, but which, by being renewed unceasingly, produced very great effects. For it must be admitted that it would be difficult to account otherwise for the slow action which two gases mutually exercise upon one another when in uniform mixture. Since their combination does not take place immediately they are mixed, and since it proceeds thereafter slowly, it is no longer brought about by their mutual affinity; it is due to a foreign cause which assists this affinity, such as light, heat, or electricity even.

Acting on these conjectures we made two mixtures, each consisting of about 1/2 litre of muriatic gas with the same volume of hydrogen gas, which we knew acted only slowly on one another; one of them we placed in complete darkness, and exposed the other to the light of the sun, which was that day very feeble. At the end of several days the first mixture was still coloured green, and appeared to have undergone no change; the second, on the contrary, had been completely decolourised in less than quarter of an hour, and was almost entirely decomposed. Being no longer able, after these experiments, to doubt as to the influence of light on the combination of the two gases, and judging from the rapidity with which it had operated, that if the light had been much more vivid it would have operated much more quickly, we made new mixtures, both of hydrogen gas with oxygenated muriatic gas and of the latter with olefiant gas, and placed them in complete darkness, awaiting some moments of bright light. Two days after having made the mixtures, we were able to expose them to the sun. Scarcely had they been exposed when they suddenly inflamed with a very loud detonation, and the jars were reduced to splinters, and projected to a great distance. Fortunately we had provided against such occurrences, and had taken precautions to secure ourselves against accident. Compound hydrogen gases would, without doubt, produce the same effect; but carbonic oxide gas placed in the same circumstances does not produce any alteration on oxygenated muriatic gas; a new proof that it does not contain hydrogen.

It is evident then, from these experiments, that it is light which effects the decomposition of oxygenated muriatic gas when it is mixed with hydrogen gas or with compound hydrogen gases. It is at least probable that, in all cases of slow and progressive action between bodies perfectly mixed, light has a very marked influence which doubtless extends also to the decomposition of animal and vegetable matters left to themselves. The action of light on colouring matters would doubtless be the same as that of a heat of 150° to 200°, and it would be interesting to prove that it is so.[2]

Again, it is quite possible that light acts on plants only as heat, with this great difference however, that heat would raise indifferently the temperature of all the particles which compose the plant, whilst light, by acting on certain particles rather than on others, just as on liquid oxygenated muriatic acid, produces an inequality of temperature doubtless very favourable to the play of the organic forces. One thing which tends to prove this is that in plants it is only the green matter which decomposes carbonic acid. This is a new point of view, from which the chemical affinities have been, as yet, but slightly studied, and we should expect important results from it, by a comparison of the action of bodies placed in darkness with that which they exercise on exposure to light.

The experiments which we have reported up till now ought to give an idea of the constitution of oxygenated muriatic gas quite different from that which had been formed of it. It had been regarded as a most easily decomposed body, and we see, on the contrary, that it resists the action of the most energetic agents, and that it is only with the help of water or of hydrogen that muriatic acid can be extracted from it in the state of gas.

We had previously discovered that dry muriates could not be decomposed by vitreous boracic acid; but seeing from our experiments that muriatic acid could not be obtained in the gaseous state except with the help of water, we again took up these decompositions with the introduction of the action of this liquid, and carried them out very easily.

We first mixed ordinary charcoal with fused muriate of silver, and exposed them to heat in a glass tube closed at one end. Before the tube became red there were disengaged abundant vapours of muriatic gas, and the silver was found reduced. We then ignited some charcoal at the most violent forge-heat which we could produce, and put ten grams of it into a porcelain retort with twice as much fused muriate of silver. When the retort was red we obtained a little muriatic acid, and an inflammable gas burning with a blue flame; but though we increased the heat to the point of destroying the retort, only 3 grams of muriate of silver were decomposed; for all the acid collected and precipitated produced only that quantity of muriate, and we found nearly all the remainder in the retort. Strongly ignited plumbago gave us similar results. The decomposition of muriate of silver by charcoal and plumbago forced to the greatest heat having been incomplete, whilst ordinary charcoal, which retains much hydrogen, effects it completely, we must conclude that it is to the hydrogen, retained by the charcoal and plumbago even at the highest temperature, that we must attribute the muriatic gas which we obtained, and that pure carbon could not decompose muriate of silver. Muriate of mercury behaves like this latter; but its volatility does not allow of exposing it to such high temperatures with charcoal.

Finally, seeing from these results that muriatic acid was not disengaged except when the water necessary for its gaseous constitution could be formed, we put fused muriate of silver and strongly ignited charcoal into a glass tube communicating with and luted to a retort containing water. The temperature having been raised to red heat, the salt was not decomposed so long as no water vapour was passed; but the moment the water began to boil, muriatic gas was disengaged very abundantly, and the decomposition of the muriate was complete in a short time. By adding only the quantity of charcoal necessary to reduce the oxide, the silver collects very well, and we do not doubt, from the great facility with which the operation takes place, that this process can be employed on the large scale in working muriate of silver ores. This salt is decomposed by means of water, even without the assistance of charcoal, but then the oxide combines with the glass and colours it yellow. Vitreous boracic acid does not decompose either fused muriate of silver, or that of baryta, or that of soda, whilst, if vapour of water is made to pass, at a red heat, over a mixture of one of these salts and boracic acid, muriatic gas is disengaged very abundantly and borates are formed.

These experiments proving that water contributes very powerfully to the disengagement of muriatic acid, we no longer doubted that it was possible to decompose muriate of soda by water, through aiding its action by that of a body which could unite with the soda. We therefore, made a mixture of two parts of white sand with one of salt and exposed it to a red heat in a porcelain tube. In this case the salt was not decomposed, but on passing a current of steam through the tube the acid was instantly disengaged in dense and very irritating fumes, and there remained in the tube a vitreous frit of soda and silica. Alumina gives results similar to those with silica; when it is perfectly dry it does not decompose salt, but with the help of water it easily disengages muriatic acid. These experiments promise happy applications, as, in giving a precise idea of the constitution of muriatic gas, they indicate the means which ought to be employed to decompose muriate of soda directly.

On seeking for the bases which could be presented to the soda to separate it from salt by means of water, we had at first thought that carbonic acid would accomplish our object, as the sub-carbonate of soda is not decomposed by heat; but we were very little satisfied with the results which we obtained. We found, in fact, that not only the sub-carbonate of soda, but also the carbonate of baryta, which cannot be decomposed by heat alone, are both decomposed with the help of water. That of lime is in the same case as the preceding; for it loses its acid by heat more easily when the action of water is made to assist. Several of these facts were known, but there was no clear idea of the manner in which they were produced, for it was even supposed that gases could produce the same effects as steam.

We cannot, as with muriatic acid, explain the disengagement of carbonic acid by water, by saying that this liquid is necessary to its gaseous constitution; for carbonic gas does not contain water, and it can quite well be disengaged from certain dry compounds, such as the carbonates of lime and lead, by heat alone; it must consequently be the affinity of water for bases which favours the disengagement of the carbonic acid.

This action of water for acids or for bases is thus much more powerful than has up till now been believed. This it is which determines the formation of sulphuric acid from sulphurous gas and oxygen gas or nitrous gas, and that of nitric acid from this latter gas and oxygen, as M. Humboldt has shown. It is again this action of water which aids the decomposition of nitrate of potash by means of clay; and it is very well known in the establishments where this decomposition is carried out on the large scale, that to obtain a large yield of acid it is necessary to employ very moist clays: when they are too dry only a very little is obtained; the acid is much more concentrated, it is true, but it comes then to too high a price. The same is the case in the decomposition of muriate of soda by clay: which takes place only by means of the water which the latter contains, and stops as soon as this is all evaporated. If certain earthy muriates have been successfully decomposed by heat, this again is only because they contain water. Thus, under whatever circumstances it is sought to obtain muriatic gas, it is only possible to succeed with the help of water. Several other acids, such as sulphuric acid and nitric acid, cannot exist in their state of greatest concentration without water, which appears to be the bond which unites their elements; but water plays a much more important part in muriatic acid. In fact, oxygenated muriatic acid is not decomposed by charcoal, and it might be supposed, from this fact and those which are communicated in this Memoir, that this gas is a simple body. The phenomena which it presents can be explained well enough on this hypothesis; we shall not seek to defend it however, as it appears to us that they are still better explained by regarding oxygenated muriatic acid as a compound body.

[1]An Alembic Club editor's note on a previous page says, "Fluoric acid gas, i.e. boron fluoride, prepared by heating fluor spar with boric anhydride."--CJG

[2]Since the reading of this Memoir we have learned that light actually does act on colouring matters in this way. [original note--CJG]

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