powers only by decoction, a process which would necessarily impair those of _Serpentaria_, connected as they are with an essential oil; whilst the addition of the acid at any other stage of the process than that directed, would produce decompositions in the vegetable substances; and it is evident that were the _Spirit of Cinnamon_ added previously, it would be entirely lost by vaporization. So in making the _Compound Decoction of Sarsaparilla_, the _Sassafras_ should be added after the other ingredients have undergone boiling. The Decoctions of _Lichen Islandicus_ and _Sarsaparilla_ constitute a popular remedy on the Continent, in certain forms of Phthisis; now it is evident that as the former plant loses its virtues by long coction, and the latter requires a protracted ebullition for the extraction of its virtues, they ought not to be included under the same general directions; each decoction should be separately performed, and the results subsequently mixed. COMPOUND MEDICINES have been divided into two Classes, _viz_. I. OFFICINAL PREPARATIONS, which are those ordered in the Pharmacopœias, and kept ready prepared in the shops. No uniform class of medicines however can answer the indications of every case, and hence the necessity of II. MAGISTRAL OR EXTEMPORANEOUS FORMULÆ. These are constructed by the practitioner at the moment, and may be either arrangements altogether new, or officinal preparations with additions, or modifications. Too much importance cannot be assigned to the Art which thus enables the physician to adopt and graduate a powerful remedy to each particular case by a prompt and accurate prescription; without this knowledge, the practitioner of the nineteenth century, with all the collateral aid of modern science, will be as helpless in the chamber of sickness as the physicians of ancient Egypt, who were obliged by the laws to follow with servile exactness the unvarying mandates of their medical code. _Extemporaneous_ are also preferable to _Officinal Formulæ_, whenever the powers of the compound are less liable to deterioration from being long kept; for examples, see _Mistura Ferri composita_; _Infusum Sennæ_; _Liquor Hydrargyri Oxymuriatis_, &c. THE CHEMICAL AND PHARMACEUTICAL ERRORS, WHICH MAY BE COMMITTED IN THE COMPOSITION OF EXTEMPORANEOUS FORMULÆ, ARE REFERABLE TO THE FOLLOWING SOURCES. 1.—_Substances are added together which are incapable of mixing, or, of forming Compounds of uniform and suitable consistence._