Much depends on the purity of the water used in the process. If it be hard pump water, the selenites with which it is impregnated will be decompounded, and the calcareous earth be deposited, after boiling; which mixing with the Magnesia will render it impure, gritty and discoloured. Rain water collected free from impurities, or clear river water, are most eligible; but if the situation of the operator does not permit him to procure these in a proper state, he should either use distilled water, which has been kept till the empyreuma is gone off, or at least such pump water as is free from any calcareous or saline impregnation. When poured on the Magnesia, it should be strained through a thick linen cloth, so as to intercept any accidental impurities which it may acquire in heating. The drying should be performed with expedition. To this end, the chalk stones should be exposed to a moderate degree of heat; and when they have been employed two or three times, should be dried before a fresh quantity of the Magnesia is put on them. Cleanliness should be particularly attended to through the whole process; and the vessels ought to be carefully covered, that no dust may enter. We may safely make use of a large copper brewing-pan, to boil the Magnesia in; for as the acid is perfectly neutralized, there can be no danger of its quitting the alkali, to which it has a greater affinity than to the metal; and copper does not readily dissolve, even in acids, when boiling hot; nor have I ever observed the least corrosion, though I have frequently used such vessels for this purpose. CHAP. II. MISCELLANEOUS OBSERVATIONS. Since the drawing up of the paper which has been the subject of the last chapter, some observations have occurred, which are either so immediately connected with, or at least deduced from it, that it may not perhaps seem inexpedient to introduce them in this place. I was very much surprised to observe, in the Lectures lately published, _as delivered_ by the very learned and ingenious Dr. Cullen, that Magnesia is there said to be no more purgative than any other absorbent earth. The sentence is this; (speaking of other absorbents) "Magnesia alba should have been added to this set. It has had a considerable reputation as an absorbent, and when neutralized, as a purgative; but I find it is not more absorbent than any of the rest, nor more purgative in less quantity, as chalk or crab's eyes given in the same quantity, viz. dramij, will have the same effect. Therefore it may be neglected."