Lectures on practical pharmacy / by Barnard S. Proctor.

Date:
1873
Catalogue details

Licence: Public Domain Mark

Credit: Lectures on practical pharmacy / by Barnard S. Proctor. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    103/502 (page 83)
    EVAPORxVTlON. 8?> tliis purpose are constructed very wide in proportion to their depth, and when set upon the fire for active ebullition, which is sometimes admissible, only the centre of the bottom of the pan is exposed to the direct action of the fire; the residue, which is apt to adhere to the pan just above the surface of the boiling liquor, is thus in a measure protected from scorching; but as a further protection, it should be frequently washed down from the sides by stirring the hot liquor, and driving it from one side to another. Evaporation over a naked fire will require pretty constant attention in all cases where an organic material is being operated upon. In most cases, at a certain stage in the process, a film appears upon the surface of the liquid which very much retards its evaporation. This is to be avoided by constant stirring, which is usually performed by hand, except in laboratories where evaporation is so constantly going on as to make it worth while having mechanical contrivances to save the time of the operator. A circular movement may be given to the stirring- rod by attaching it to a wheel moved by machinery, but as this moves in one direction at a constant rate it is not nearly so effective as stirring by the hand, which moves the fluid in all directions, and most effectually breaks the pellicle. A ^ better arrangement for meclianical stirring is to have the ^ stirrer hinged above the pan, and moved backward and for- ' ward, the bottom of the stirrer being like a broad spade almost [ fitting the bottom of the pan, and the hinge being in the . focus of the pan, or rather in the centre of the imaginary I sphere of which the pan is a segment; the blade of the stirrer having larger perforations or notches through which the liquor will flow while the stirrer moves from side to side. The cut exhibits Mr Eeynolds’s system of stirring, as described at the Pharmaceutical Conference, 1867. His description will be found in ‘‘Pharmaceutical Journal ” 1866-7, page 221. The material of the pan should be copper, block tin, or enamelled iron; the first and last being preferred for