Hoefer draws attention to a remarkable observation recorded by this alchemist.  Speaking of the “spirit of mercury,” Basil Valentine says it is “the origin of all the metals; that spirit is nothing else than an air flying here and there without wings; it is a moving wind, which, after it has been chased from its home of Vulcan (that is, fire), returns to the chaos; then it expands and passes into the region of the air from whence it had come.”  As Hoefer remarks, this is perhaps one of the earliest accounts of the gas discovered by Priestley and studied by Lavoisier, the gas we now call oxygen, and recognise as of paramount importance in chemical reactions.

[Illustration:  FIG.  XII. See p. 92.]

Besides discovering and recording many facts which have become part and parcel of the science of chemistry, the alchemists invented and used various pieces of apparatus, and conducted many operations, which are still employed in chemical laboratories.  I shall reproduce illustrations of some of these processes and pieces of apparatus, and quote a few of the directions, given in a book, published in 1664, called The Art of Distillation, by John French, Dr. in Physick.

The method recommended by French for hermetically sealing the neck of a glass vessel is shown in Fig.  VI. p. 80.  The neck of the vessel is surrounded by a tray containing burning coals; when the glass melts it is cut off by shears, and then closed by tongs, which are made hot before use.

Fig.  VII. p. 81, represents a method for covering an open vessel, air-tight, with a receptacle into which a substance may be sublimed from the lower vessel.  The lettering explains the method of using the apparatus.

French gives very practical directions and much sound advice for conducting distillations of various kinds.  The following are specimens of his directions and advice:—­

  “When you put water into a seething Balneum wherein there are
  glasses let it be hot, or else thou wilt endanger the breaking of
  the glasses.

  “When thou takest any earthen, or glass vessel from the fire,
  expose it not to the cold aire too suddenly for fear it should
  break.

“In all your operations diligently observe the processes which you read, and vary not a little from them, for sometimes a small mistake or neglect spoils the whole operation, and frustrates your expectations.

  “Try not at first experiments of great cost, or great difficulty;
  for it will be a great discouragement to thee, and thou wilt be
  very apt to mistake.