Your first conjecture is very right: but you are mistaken in the last; for nothing will be left in the cup. The white vapour is the oxygenated sulphur, which assumes the form of an elastic fluid of a pungent and offensive smell, and is a powerful acid. Here you see a chemical combination of oxygen and sulphur, producing a true gas, which would continue such under the pressure and at the temperature of the atmosphere, if it did not unite with the water in the plate, to which it imparts its acid taste, and all its acid properties. --You see, now, with what curious effects the combustion of sulphur is attended. CAROLINE. This is something quite new; and I confess that I do not perfectly understand why the sulphur turns acid. MRS. B. It is because it unites with oxygen, which is the acidifying principle. And, indeed, the word _oxygen_ is derived from two Greek words signifying _to produce an acid_. CAROLINE. Why, then, is not water, which contains such a quantity of oxygen, acid? MRS. B. Because hydrogen, which is the other constituent of water, is not susceptible of acidification. --I believe it will be necessary, before we proceed further, to say a few words of the general nature of acids, though it is rather a deviation from our plan of examining the simple bodies separately, before we consider them in a state of combination. Acids may be considered as a peculiar class of _burnt_ bodies, which during their combustion, or combination with oxygen, have acquired very characteristic properties. They are chiefly discernible by their sour taste, and by turning red most of the blue vegetable colours. These two properties are common to the whole class of acids; but each of them is distinguished by other peculiar qualities. Every acid consists of some particular substance, (which constitutes its basis, and is different in each,) and of oxygen, which is common to them all. EMILY. But I do not clearly see the difference between acids and oxyds. MRS. B. Acids were, in fact, oxyds, which, by the addition of a sufficient quantity of oxygen, have been converted into acids. For acidification, you must observe, always implies previous oxydation, as a body must have combined with the quantity of oxygen requisite to constitute it an oxyd, before it can combine with the greater quantity that is necessary to render it an acid. CAROLINE. Are all oxyds capable of being converted into acids? MRS. B.