Chap. I. Properties. we begin to collect the gas, that we may be sure that all the gas 1. It is invisible like common air, and capable like it of indefinite contraction and expansion. Its smell is peculiar, and when let into common air it becomes visible by forming a white smoke in consequence of the avidity with which it absorbs It reddens vegetable blues and has a very sour taste, and possesses powerful corrosive qualities. moisture. 2. I found its specific gravity at the temperature of 60° and when the barometer stood at 30 inches 12843. The true specific gravity has been shown by Gay-Lussac to be the mean of the specific gravity of hydrogen gas and chlorine gas, or 1-2847. 3. Animals are incapable of breathing it; and when plunged into jars filled with it, they die instantaneously in convulsions. Neither will any combustible burn in it. It is remarkable, however, that it has a considerable effect upon the flame of combustible bodies; for if a burning taper be plunged into it, the flame, just before it goes out, may be observed to assume a green colour, and the same tinge appears the next time the taper is lighted.* 4. Its refracting power, according to the experiments of Chevreul, is 1.527, that of air being unity.† 5. Water absorbs this gas with prodigious rapidity. If we fill a phial with it, and take out the stopper while the mouth of the phial is under water, the liquid rushes in with fully as much violence as it would into a vacuum. At the temperature of 69° one cubic inch of water is capable of absorbing 417·822 cubic inches of muriatic acid gas. The temperature of the liquid increases considerably; and its volume at 69° is 13438 cubic inch. It is obvious from this that 100 grains of acid of this strength contain 103 cubic inches of the acid gas; and a cubic inch of this acid contains 311-04146 cubic inches of the acid gas. Acid of this strength has a specific gravity of 1·1958, and contains 40-39 per cent. of real acid united with 59-61 of water. In winter liquid acid may be obtained having a specific gravity of 1·212, and probably by artificial cooling the saturation of the * Priestley, ii. 293. † Ann. de Chim. et de Phys. xxxi. 166. water might be carried still farther. The following table which I constructed with considerable care, from actual experiment,* exhibits the composition of liquid muriatic acid of various specific gravities. Class II. Ice also absorbs this gas, and is at the same time liquefied. The quantity of this gas absorbed by water diminishes as the heat of the water increases, and at a boiling heat water will not absorb any of it. When water impregnated with it is heated, the gas is again expelled unaltered. Hence muriatic acid gas may be procured by heating the common muriatic acid of commerce. It was by this process that Dr. Priestley first obtained it. Liquid muriatic acid, as the solution of the gas in water is Preparation, called, being more employed in chemical experiments than any other acid, requires to be obtained quite free from all impurity; and as the muriatic acid of commerce does not possess that indispensable requisite, it is requisite to prepare it in the laboratory. The apparatus employed for this purpose is represented in the figure in next page. A is a tubulated retort of such a size that a couple of pounds of common salt may be put into it without filling it more than half full. The beak of this retort passes through a perforated cork into the opening of the side of the jar B. This jar has two mouths. Through one of them the tube of safety a passes to the very bottom of the jar. It * First Principles, i. 87. passes through a perforated cork, so as to be air-tight in the mouth of the jar. About an inch of water is put into the jar B, into which the tube of safety a plunges. From the other mouth of the jar B issues the glass tube b, also fixed tight in the jar by means of a perforated cork. This tube bending at right angles passes into one of the three mouths of the tall jar C filled about two-thirds with water. The jar C has three mouths. Into the first passes the glass tube b; into the second the tube of safety d; and into the third the glass tube e. This tube passes to the bottom of another Woolfe's bottle D, which has also its tube of safety g, and the bent tube E, and which like the jar C is filled two-thirds with distilled water. All the mouths of the Woolfe's bottles must be carefully luted with fat lute, and then a piece of cloth dipt into hot glue is to be laid over the joinings and firmly tied on with a string. This, together with the corks, enables the apparatus to withstand the requisite pressure without allowing the gas to escape. The bent tubes b and e would be very apt to break should the apparatus undergo any agitation. On this account there is a joint in each at m, made by having two tubes instead of one, the two extremities of which are brought nearly in contact and tied together by means of a slip of caoutchouc. I take a ribbon of caoutchouc and dipping it into caoutchouc varnish, make the sides overlap each other, and then tie it in that situation and allow it to dry. It will now form a caoutchouc tube through which the muriatic acid cannot penetrate.* * * These caoutchouc joints are exceedingly convenient. The thin flat sheets of caoutchouc sold in London answer best. These joints do not answer so A After the apparatus is properly luted, sulphuric acid is to be poured by little at a time through the tubular mouth into the retort A.* This may be either done by simply taking out the stopper and replacing it as quickly as possible after the acid has been poured in. Or we may fix the bent funnel A into the mouth of the retort, making it air tight by passing it through a perforated cork. After the sulphuric acid has passed through this funnel, a portion of it remains at the bending m, which effectually prevents the muriatic gas from escaping that way. The advantage of using such a bent funnel is, that we may add sulphuric acid as it is wanted, without deranging the apparatus or stopping the process. The small quantity of water at the bottom of jar B is first saturated with the gas; then it passes by the tube b into the jar C, and is absorbed by the water in that jar. As the temperature of the water is much raised by this absorption, there is an advantage in surrounding the jar C with cold water, or still better with a mixture of water and ice, when ice can be procured. In proportion as the water absorbs the gas, its bulk increases, and by the time that it is saturated we shall find the jar C nearly filled with liquid muriatic acid. If we continue the process after the water in C is saturated, the gas will pass over into D, and saturate the water which it contains. In this way we may saturate the water in as many Woolfe's bottles as we please, supposing them attached to each other in the way the D is to C. When the water in C is saturated with muriatic acid gas, its colour is always a light yellow. If we continue the current of gas after the water is saturated, this yellow colour gradually diminishes and at last disappears, or rather the colour is conveyed to the jar D, the water in which in its turn becomes yellow. In this way we may drive the yellow colour from one jar to another as far as we please. The colouring matter, whatever it may be, is very minute in quantity. If we add to well when ammoniacal gas is passed through them. Because caoutchouc absorbs ammonia, and is rendered soft by it, so that it soon gives way. The best proportions are 7.5 salt, 9.2 sulphuric acid, 1 water. Class II. Chap. I. the coloured liquid a very minute particle of tin, it will be dissolved and the colour disappears. I suspect that the colouring matter is bromine; but I have no other proof except the certainty that bromine exists in minute quantity in common salt, and that a very small portion of bromine is capable of communicating a similar colour to colourless muriatic acid. Muriatic acid thus obtained has a strong pungent smell similar to the gas: it emits a white smoke when exposed to the air, which becomes much more conspicuous if a little ammonia be placed in the neighbourhood of the acid. The following table drawn up by Mr. Dalton gives us the boiling point of this acid at various densities. Composition. We see from this table that the boiling point is a maximum when the liquid is a compound of 1 atom acid and 16 atoms water; or when the liquid contains about 20 per cent. of acid. The boiling point of acid of every other strength is lower. Acid containing about 30 per cent, of real acid boils at the same temperature as water. If the acid be stronger its boiling temperature is lower than that of water. The boiling point of acid of 1203 is 107°. 6. It has been demonstrated that muriatic acid gas is a compound of equal volumes of hydrogen and chlorine gases, united together without any alteration in bulk.* Hence its atomic weight is 4:625. 7. Mr. Faraday ascertained that muriatic acid gas is condensed into a colourless liquid by a pressure of 40 atmospheres, at the temperature of 50°† See vol. i. p. 107 + Phil. Trans. 1826, p. 544. |