nails in yet another way, we may proceed to subject them to a _chemical_ change, by an experiment very simple in itself, but involving certain precautions, by neglect of which the tyro in chemical operations would incur some personal risks; these might however be obviated by using only very small quantities of the materials (a mere pinch of iron filings and a few drops of sulphuric acid), when the results would still be sufficiently observable. A few of the iron nails having been placed in a flask of thin glass, we pour upon them a mixture of oil of vitriol (sulphuric acid) and water, which has previously been prepared by gradually adding 1 measure of the acid to 5 measures of water. The action that takes place is greatly accelerated by heat, and indeed the contents should be heated to boiling by standing the flask on a layer of fine sand spread on an iron plate and gently heated from below. The nails will soon disappear, being completely dissolved by the acid liquid, and the turbid solution should be filtered through filtering paper as rapidly as possible and while still hot. This turbid and dirty looking condition is due to foreign matters in the nails, for these never consist of _pure_ iron. The filtered liquid is set aside to cool in a closed vessel, in which after a time will be found a deposit of crystals of a pale bluish-green colour. The liquor above these having been poured off, the crystals are to be rinsed with a very small quantity of _cold_ water, and then dried between folds of blotting-paper, after which they are ready for examination. The quantity of the diluted acid put into the flask should have a certain proportion to the weight of the nails; about 5 _fluid ounces_ to 1 ounce of iron will be found convenient, for if less is used the nails will not be entirely dissolved, and an excess will tend to keep the crystals in solution instead of depositing them when cold. The nails—as such—will now have passed out of existence: can we say that the iron that formed them exists in the crystals? Certainly not as the _metal_ iron, for every property of the metal will have disappeared. The crystals are brittle, can be crushed in a mortar—they are translucent—they show no metallic lustre, but only glassy surfaces—they are readily dissolved by water—they are not attracted by a magnet. The most powerful lens will fail to show the least particle of iron in them; they have in their properties no assignable relation to the metal of the nails, but are matter of quite another sort; and be it noted that this entire _otherness_ is the special and characteristic sign of _chemical change_. So complete is the transformation in the case we have been considering that it would never have been said that iron was contained in these crystals, but rather that the metal had for ever passed out of existence, but for one circumstance; and that is, that by subjecting the crystals to certain processes of _chemical analysis_ we can again obtain