“The illustrious discoverer of the oxymuriatic acid considered it as muriatic acid freed from hydrogen; and the common muriatic acid as a compound of hydrogen and oxymuriatic acid; and on this theory he denominated oxymuriatic acid dephlogisticated muriatic acid.
“M. Berthollet, a few years after the discovery of Scheele, made a number of important and curious experiments on this body; from which he concluded that it was composed of muriatic acid and oxygen; and this idea for nearly twenty years has been almost universally adopted.”
Having thus accurately stated the position, he proceeds to attack it. In the first place, he points out that Henry, ten years before, had shown that hydrogen could be produced from muriatic acid gas by the agency of electricity; this hydrogen was assumed by Henry to be due to water contained in the gas. Davy, in his Bakerian lecture of 1808, had shown that muriatic acid gas gave hydrogen when treated with potassium, and he had stated “that muriatic acid can in no instance be procured from oxymuriatic gas, or from dry muriates, unless water or its elements be present.”
Gay Lussac and Thenard had concluded “that muriatic acid gas contains about one-quarter of its weight of water; and that oxymuriatic acid is not decomposable by any substances but hydrogen, or such as can form triple combinations with it.”
He then points out, what he had already stated in a former paper, that charcoal freed from hydrogen and moisture by intense ignition in vacuo may be heated to whiteness by the voltaic battery in oxymuriatic or muriatic acid gases without affecting any change in them.
It now occurred to him that if the liquor of Libavius (stannic chloride) is a combination of muriatic acid and oxide of tin, as then surmised, oxide of tin ought to be separated from it by means of ammonia. On admitting ammonia gas to the tin chloride over mercury, the substances combined with great heat, a white solid was obtained; “some of it was heated to ascertain if it contained oxide of tin, but the whole volatilised, producing dense pungent fumes.” The experiment was repeated with every care, but no oxide of tin could be obtained.
He was next led to study the behaviour of ammonia with the substances he had formerly obtained, by the action of oxymuriatic gas on phosphorus (see p. [129]). One of these is solid, and is now known as phosphorus pentachloride; the other is liquid, and is termed phosphorus trichloride.
“The first,” he says, “on the generally received theory of the nature of oxymuriatic acid, must be considered as a compound of muriatic acid and phosphoric acid. It occurred to me that if the acids of phosphorus really existed in these combinations, it would not be difficult to obtain them, and thus to gain proof of the existence of oxygen in oxymuriatic acid.”
He therefore brought ammonia gas into contact with the solid compound of oxymuriatic acid and phosphorus. Much heat was produced, and a white opaque powder was formed.
“Supposing that this substance was composed of the dry muriate and phosphate of ammonia; as muriate of ammonia is very volatile, and as ammonia is driven off from phosphoric acid, by a heat below redness I conceived that by igniting the product obtained I should procure phosphoric acid ... but found to my great surprise that it was not at all volatile nor decomposable at this degree of heat, and that it gave off no gaseous matter. The circumstance that a substance composed principally of oxymuriatic acid and ammonia should resist decomposition or change at so high a temperature induced me to pay particular attention to the properties of this new body.”