§ =28.= We will conclude this chapter with some few remarks regarding the practical methods of the alchemists. In their experiments, the alchemists worked with very large quantities of material compared with what is employed in chemical researches at the present day. They had great belief in the efficacy of time to effect a desired change in their substances, and they were wont to repeat the same operation (such as distillation, for example) on the same material over and over again; which demonstrated their unwearied patience, even if it effected little towards the attainment of their end. They paid much attention to any changes of colour they observed in their experiments, and many descriptions of supposed methods to achieve the _magnum opus_ contain detailed directions as to the various changes of colour which must be obtained in the material operated upon if a successful issue to the experiment is desired.[40] In plates 5 and 6 we give illustrations of some characteristic pieces of apparatus employed by the alchemists. Plate 5, fig. A, and plate 6, fig. A, are from a work known as _Alchemiae Gebri_ (1545); plate 5, fig. B, is from Glauber’s work on Furnaces (1651); and plate 6, fig. B, is from a work by Dr. John French entitled _The Art of Distillation_ (1651). The first figure shows us a furnace and alembics. The alembic proper is a sort of still-head which can be luted on to a flask or other vessel, and was much used for distillations. In the present case, however, the alembics are employed in conjunction with apparatus for subliming difficultly volatile substances. Plate 5, fig. B, shows another apparatus for sublimation, consisting of a sort of oven, and three detachable upper chambers, generally called aludels. In both forms of apparatus the vapours are cooled in the upper part of the vessel, and the substance is deposited in the solid form, being thereby purified from less volatile impurities. Plate 6, fig. A, shows an athanor (or digesting furnace) and a couple of digesting vessels. A vessel of this sort was employed for heating bodies in a closed space, the top being sealed up when the substances to be operated upon had been put inside, and the vessel heated in ashes in an athanor, a uniform temperature being maintained. The pelican, illustrated in plate 6, fig. B, was used for a similar purpose, the two arms being added in the idea that the vapours would be circulated thereby.