THE OPERATIVE CHEMIST. 
774 
the essential oil of the grape, and depraves the taste of the 
spirit, it is proper to add limestone to separate this acid ; or, 
which is still better, quick lime, which will also absorb 
some of the oil. 
The thick grouty matter left in these distillations, or the 
cake left on pressing the lees, is dried and burned for its 
alkali, which is sold by the name of cendres gravellées, and 
is esteemed by the French dyers as preferable to potash or 
pearlash. 
The physeter of Mr. Field, would be very useful in the 
manufacture of common brandy from these grouty materials. 
Fig. 359, represents a section of the physeter, or percolator; and fig. 
360, is the elevation of the same. A, is a strong tub, with iron hoops, 
moveable upon castors. B, is a wooden hoop, fixed on the inside,’a few 
inches from the bottom. C, is a strainer, resting upon the hoop, b ; that 
represented in the figure is a sieve of copper wire. D, is a small lifting 
pump, placed on the outside of the tub, and communicating with the 
lower part. E, in the side figure, shows the copper sieve, covered first 
with a baize strainer, and over that another of the silk called lute-string, 
to show how the edges of these are secured by being turned over the rim 
of the strainer ; the space left between them and the side of the tub, is 
secured by a list of woollen cloth, and the whole fastened down by the 
hoop, f, which fits so closely to the sides of the tub when driven down, 
as to render the whole air tight. G, is a funnel cock, by which the lower 
part of the tub may, if required, be filled with water. H, is another 
cock, by which the air in the lower part may be let dut while it is filling 
with water, by the cock, g; or the air let in it, if the purpose for which it 
was exhausted by the pump is answered. 
Sheets of filtering paper may be placed between the cloths of the 
strainer, or the number of these may be increased ; and if a copper wire 
strainer is judged improper, a wooden grating may be substituted. 
As the action of the pump occasions the atmosphere to 
press upon the surface of the liquor, at the rate of 14 
pounds to every square inch of surface, this apparatus is 
very powerful in forcing liquids through several layers of 
filters, or filtering powders, or in draining the last portions 
of liquor from spongy substances which would otherwise 
retain a large portion of the liquid with which they are 
impregnated. 
Potatoe Spirit. 
The potatoes best adapted for distillation, are those that 
contain the most fecula; of which, in an average, they con 
tain one quarter of their weight. It was a great incon 
venience in their use, that they could at first only be em 
ployed from October to May; but means have now been 
found to avoid this defect. 
The first method is, to boil the potatoes by steam ; this