That which is made with the vitriolic acid, may be condensed into crystals similar in all respects to epsom-salt. That which is made with the nitrous is of a yellow colour, and yields saline crystals, which retain their form in a very dry air, but melt in a moist one. That which is produced by means of spirit of salt, yields no crystals; and if evaporated to dryness, soon melts again when exposed to the air. That which is obtained from the union of distilled vinegar with _magnesia_, affords no crystals by evaporation, but is condensed into a saline mass, which, while warm, is extremely tough and viscid, very much resembling a strong glue both in colour and consistence, and becomes brittle when cold. By these experiments _magnesia_ appears to be a substance very different from those of the calcarious class; under which I would be understood to comprehend all those that are converted into a perfect quick-lime in a strong fire, such as _lime-stone_, _marble_, _chalk_, those _spars_ and _marles_ which effervesce with aqua fortis, all _animal shells_ and the bodies called _lithophyta_. All of these, by being joined with acids, yield a set of compounds which are very different from those we have just now described. Thus, if a small quantity of any calcarious matter be reduced to a fine powder and thrown into spirit of vitriol, it is attacked by this acid with a brisk effervescence; but little or no dissolution ensues. It absorbs the acid, and remains united with it in the form of a white powder, at the bottom of the vessel, while the liquor has hardly any taste, and shews only a very light cloud upon the addition of alkali.[4] The same white powder is also formed when spirit of vitriol is added to a calcarious earth dissolved in any other acid; the vitriolic expelling the other acid, and joining itself to the earth by a stronger attraction; and upon this account the _magnesia_ of sea-water seems to be different from either of those described by _Hoffman_. He says expressly, that the solutions of each of his powders, or, what is equivalent, that the liquors from which they are obtained, formed a coagulum, and deposited a white powder, when he added the vitriolic acid;[5] which experiment I have often tried with the marine bittern, but without success. The coagulum thus formed in the mother of nitre may be owing to a quantity of quick-lime contained in it; for quick-lime is used in extracting the salt-petre from its matrix. But it is more difficult to account for the difference between _Hoffman's_ bittern and ours, unless we will be satisfied to refer it to this, that he got his from the waters of salt springs, which may possibly be different from those of the sea.