It is not surprising that less light and heat should be liberated in this than in almost any other combustion, since the oxygen, instead of entering into a solid or liquid combination, as it does in the phosphoric and sulphuric acids, is employed in forming another elastic fluid; it therefore parts with less of its caloric. EMILY. True; and, on second consideration, it appears, on the contrary, surprising that the oxygen should, in its combination with carbon, retain a sufficient portion of caloric to maintain both substances in a gaseous state. CAROLINE. We may then judge of the degree of solidity in which oxygen is combined in a burnt body, by the quantity of caloric liberated during its combustion? MRS. B. Yes; provided that you take into the account the quantity of oxygen absorbed by the combustible body, and observe the proportion which the caloric bears to it. CAROLINE. But why should the water, after the combustion of carbon, rise in the receiver, since the gas within it retains an aëriform state? MRS. B. Because the carbonic acid gas is gradually absorbed by the water; and this effect would be promoted by shaking the receiver. EMILY. The charcoal is now extinguished, though it is not nearly consumed; it has such an extraordinary avidity for oxygen, I suppose, that the receiver did not contain enough to satisfy the whole. MRS. B. That is certainly the case; for if the combustion were performed in the exact proportions of 28 parts of carbon to 72 of oxygen, both these ingredients would disappear, and 100 parts of carbonic acid would be produced. CAROLINE. Carbonic acid must be a very strong acid, since it contains so great a proportion of oxygen? MRS. B. That is a very natural inference; yet it is erroneous. For the carbonic is the weakest of all the acids. The strength of an acid seems to depend upon the nature of its basis, and its mode of combination, as well as upon the proportion of the acidifying principle. The same quantity of oxygen that will convert some bodies into strong acids, will only be sufficient simply to oxydate others. CAROLINE. Since this acid is so weak, I think chemists should have called it the _carbonous_, instead of the _carbonic_ acid. EMILY. But, I suppose, the carbonous acid is still weaker, and is formed by burning carbon in atmospherical air. MRS. B. It has been lately discovered, that carbon may be converted into a gas, by uniting with a smaller proportion of oxygen; but as this gas does not possess any acid properties, it is no more than an oxyd; it is called _gaseous oxyd of carbon_. CAROLINE. Pray is not carbonic acid a very wholesome gas to breathe, as it contains so much oxygen? MRS. B.