Chymists cannot perform the operations of their art without the help of a considerable number of vessels, instruments, and furnaces, adapted to contain the bodies on which they intend to work, and to apply to them the several degrees of heat required by different processes. It is therefore proper, before we advance to the operations themselves, to consider particularly and minutely what relates to the instruments with which they are to be performed. Vessels intended for Chymical Operations should, to be perfect, be able to bear, without breaking, the sudden application of great heat and great cold; be impenetrable to every thing, and unalterable by any solvent; unvitrifiable, and capable of enduring the most violent fire without melting: but hitherto no vessels have been found with all these qualities united. They are made of sundry materials; namely, of metal, of glass, and of earth. Metalline vessels, especially those made of Iron or Copper, are apt to be corroded by almost every saline, oily, or even aqueous substance. For this reason, in order to render the use of them a little more extensive, they are tinned on the inside. But, notwithstanding this precaution, they are on many occasions not to be trusted; and should never be employed in any nice operations which require great accuracy: they are, moreover, incapable of resisting the force of fire. Earthen vessels are of several sorts. Some, that are made of a refractory earth, are capable of being suddenly exposed to a strong fire without breaking, and even of sustaining a great degree of heat for a considerable time: but they generally suffer the vapours of the matters which they contain, as well as vitrified metals, to pass through them, especially the glass of lead, which easily penetrates them and runs through their pores as through a sieve. There are others made of an earth that, when well baked, looks as if it were half vitrified: these being much less porous, are capable of retaining the vapours of the matters which they contain, and even glass of lead in fusion; which is one of the severest trials a vessel can be put to: but then they are more brittle than the other sort. Good glass vessels should constantly be employed in preference to all others, whenever they can possibly be used: and that not only because they are no way injured by the most active solvents, nor suffer any part of what they contain to pass through, but also because their transparency allows the Chymist to observe what passes within them: which is always both curious and useful. But it is pity that vessels of this sort should not be able to endure a fierce fire without melting. We shall take care, when we come to describe the several sorts of chymical instruments, and the manner of using them, to note what vessels are to be preferred to others on different occasions.