_They excite a particular sensation on the palate, which we call sour. They change the blue colour of vegetables red._ All of them, except the carbonic acid, effervesce with the _volatile_ as well as the _fixed alkalies when in the state of carbonates, as they are most commonly found in commerce_. Oxygen is the principle of almost all acids; their difference depends upon the base combined with the oxygen: thus oxygen combined with carbon or pure charcoal, forms _carbonic acid_; with nitrogen the _nitric acid_; with sulphur the _sulphuric acid_, _&c._ _&c._ _Gas_ is a term implying the same as _air_; but as the term air, when used, is liable to be misunderstood for the air of the atmosphere, which is, as we have seen, a compound body, the term gas is more appropriately applied to all elastic fluids of a specific kind. Thus we say _carbonic acid gas_, _oxygenous gas_. The difference between carbonic acid and carbonic acid gas, and oxygen and oxygenous gas, consists in the latter being combined with _heat_ only, and in the state of air, while in the former they are fixed in some body, as in carbonate of pot-ash and oxide of lead, in both which cases the carbonic acid exists in a fixed state, or combined with the pot-ash, and the oxygen is in a fixed state, or combined with the lead. We may now treat of _carbonic acid gas_, which is thus produced, as well as in many other ways: when charcoal is burned in oxygen gas, exactly sufficient for its combustion, both the charcoal and oxygen disappear, and an elastic fluid is found in the vessel, which is equal in weight to both. This air or gas is carbonic acid gas; it combines with lime, the alkalies, and pure or burnt magnesia: it constitutes a considerable portion of the weight of chalk, limestone and marble, as is readily seen by comparing these bodies before and after their conversion into quicklime. It is frequently combined with hydrogen. The gas with which the streets are now lighted is chiefly carburetted hydrogen. Carbonic acid gas has the following properties. It extinguishes flame, and, like nitrogen and hydrogen, kills animals immersed in it. It is heavier than common air, and may therefore be poured out of one vessel into another like water. Cider, wine, beer and other fermented liquors owe their briskness to the carbonic acid which they contain; soda-water also owes its briskness entirely to the quantity of carbonic acid gas which it contains, a small quantity of heat being sufficient to give the acid the gaseous state.