THE DISCOVERY OF GASES - Black 2#


Black
Joseph Black was born in Bordeaux at 1728. Black was educated in medicine at Glasgow University. In June 1754, Black presented his inaugural disertation : De Humore Acido a Cibis orto, et Magnesia Alba (“On the acid humour arising from food and magnesia alba”). This has an interesting appendix of chemical experiments with an explanation of the relation between mild and caustic alkalis. In 1756, the Philosophical Society of Edinburg publihed his dissertation as “Experiments upon Magnesia alba, Quicklime and some other Alcaline Substances”.
Black showed that when magnesia alba (basic carbonate, xMgCO3, yMg(OH)2, zH2O) is heated, a gas is evolved, which he called fixed air (Note : it was identical with van Helmont’s gas sylvestre). The residue of calcined magnesia (MgO) is lighter than the magnesia alba and more alkaline. By weighing the magnesia alba in a glass retort and heating, Black found that, “of the volatile parts in that powder, a small proportion only is water, ... the volatile matter lost in the calcination of magnesia alba is mostly air and hence the calcined magnesia does not emit air, or make an effervescence, when mixed with acd”. Thus :
magnesia alba = calcined magnesia + water + fixed air
magnesia alba + acid = magnesia salt + fixed air

calcined magnesia + acid = magnesia salt
In his experiment on limestone and quicklime, he found that limestone effervescence with acids, giving off fixed air. On heating, it forms quicklime and only a trace of water ; the considerable loss in weight must be due to the escape of fixed air.
limestone = quicklime + fixed air (1)
There were three alkalis recognizd in Black’s time : vegetable (potash), marine (soda) and volatile (ammonia), and a mild and caustic form of each. The mild form (carbonate) was known to be converted into the caustic form (hydroxide) by treatment with slaked lime.
Black found that if a given weight of limestone is converted into quicklime according to equation (1), the quicklime slaked with water, and the slaked lime boiled with a solution of mild alkali (he used potassium carbonate), the alkali becomes caustic and the original weight of limestone is recovered. Thus :
Quicklime + mild alkali =limestone + caustic alkali (2)
Equations (1) and (2) show that :
mild alkali = caustic alkali + fixed air (3)
When limestone is dissolved in acid, fixed air is evolved with effervescence and salt is formed. On addition of mild alkali to the solution, the original weight of limestone is reprecipitated and there is no effervescence because the fixed air of the mild alkali joins to the lime to produce limestone. Caustic alkali contains no lime or a mere trace which had been dissolved by water.

Figure 16. A chemical laboratory in 1765

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