906 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
[May 13,1871. 
stances from the filtering medium. But the process 
being mainly one of filtration, your reporter ventures to 
present a few suggestions which he deems as applicable 
to pharmaceutical percolation as to filtration proper. 
Filters or percolators should be constructed tall and 
narrow, so that the filtering or exhausting liquid should 
have to travel a great distance through a large mass of 
filtering material, or powdered drugs in the case of per¬ 
colation, as only with the increased contact and surface 
the efficiency of a filter is increased. If of two filters of 
the same capacity, the one is twice as tall as the other, 
the tallest will be almost twice as effective, since in the 
shorter filter the liquid passes but half as long through 
the filtering material, withdrawing itself soon from the 
same, while each particle of liquid comes in contact with 
but half the quantity of filtering material, the paths 
which the filtering-liquid passes, being increased in 
number in the same ratio as they are decreased in length. 
Although used in the arts for many years, the prac¬ 
tical application of this principle in pharmacy is of but 
recent date; namely, the use of fractional percolation, 
an apparatus for the same, or a series of filters, being in 
fact no more than a long tall filter cut into many short 
filters for the sake of more convenient handling, and more 
convenient separation of the exhausted part of the filter 
in the first one of the series. Here I would beg leave to 
remark that various suggestions in regard to fractional 
percolation, dictating the exchange of different filters in 
an especial order, with retention of some of the first fil¬ 
trate (percolate), prior to its passage through all of the 
filtering material (powdered drug) appears to me arbi¬ 
trary, since no percolate, no matter how concentrated, is 
so strong but that its passage through some fresh por¬ 
tions of unused drugs would increase its strength, the 
active principle of that part of the drug being exhausted 
at the same time to some extent, thus rendering more 
complete the work of exhaustion of subsequent portions 
of filtering liquid. 
Fractional percolation, no matter whether two or an 
indefinite number of filters are used, should be so con¬ 
ducted that each particle of the filtering-liquid would 
pass through all of the filtering material. This will 
■ensure a thorough exhaustion of the filtering material, 
and a corresponding thorough purification (in the case of 
percolation through saturation) of the filtering liquid. 
Another point worthy of consideration is the difference 
in the results between upward and downward filtration. 
In the latter, the liquid, following its own gravity only, 
will select those paths where it finds the least resistance, 
namely, around the single particles of the filtering ma¬ 
terial, which to a great extent it will leave untouched 
and therefore unused. In upward filtration, on the other 
hand, the passage of the liquid is guided by hydrostatic 
pressure in a straight upward direction regardless of re¬ 
sistance, and the filtering material is more thoroughly 
exhausted. For liquids of low specific gravity, like oils, 
this mode of filtration is well established, because the 
water used in displacing the last portions of the oil is 
quite effective, consequent upon the difference of the 
specific gravity of the two liquids. But if an extract, a 
syrup, or other liquid, heavier than water, is to be dis¬ 
placed by that liquid, a partial diffusion of the same 
through water will take place. This is quite trifling if 
the liquid is well followed up by the displacing fluid 
(water), but has been deemed sufficient by some to form 
an obstacle to the practical application of upward 
filtration. 
In experiments made in this direction, your reporter 
found that syrup weighing thirty-five degrees Beaume, 
displaced by water, soon yielded a filtrate of but two de¬ 
grees Beaume, if the filter was kept at a temperature of 
about ninety degrees F., to ensure fluidity of the syrup; 
the filtering material in this case was bone-black. 
Where the thorough exhaustion of the last trace is of 
great significance, the filter, which for pharmaceutical 
purposes is rarely of great dimensions, might be made 
revolvable around an axis, so that top or bottom could 
be interchanged for the reception or discharge of the 
liquid filtered. Here the filtration might be carried on 
upwards, and the exhaustion of the filter downwards. 
Pressure filtration, as also that with the exclusion of air, 
can be carried on well and simply in the manner of up¬ 
ward filtration. 
As to the material most useful for filtration to the 
pharmacist, I will mention besides filtering-paper, cotton 
batting for most ordinary liquids, for which might be 
substituted in many cases white clay, fuller’s earth, when 
only turbidity is the motive for filtration. 
In several instances it has come to the notice of your 
reporter that otherwise good pharmacists, who never 
would buy or make an inferior preparation, would filter 
liquids, like ferrated elixir of Calisaya, through large 
quantities of bone-black. The great absorbent power of 
bone-black was completely lost sight of. A plug of cotton 
batting would have been more appropriate. If bone- 
black is used for filtering pharmaceutical preparations, 
it should be completely freed from its phosphates by 
muriatic acid, the nitrogenous carbon remaining behind 
being a much more powerful substance for filtration 
(decolorization) than the ordinary bone-black, while it 
contains nothing soluble that would contaminate any 
pharmaceutical preparation. 
For chromic acid, or similar oxidizing agents or caus¬ 
tics, the proper filtering medium is glass-powder or gun¬ 
cotton, while for the filtration of mercury a double layer 
of good chamois skin should be taken, and gentle pres¬ 
sure applied to effect the passage of the metal.— Pro¬ 
ceedings of the American Pharmaceutical Association . 
TESTING COCHINEAL. 
BY J. M. MERRICK. 
I give in the following article the outlines of the 
method I am in the habit of using for testing samples 
of cochineal to ascertain their comparative colouring 
powers. I have not seen it described in print, and while 
it is a much closer and more accurate method than that 
which is based upon dyeing strips of mordanted woollen 
stuffs, it is preferable to the bleaching with chloride of 
lime method,—as the oxidizing substance used, viz, 
potassic permanganate, does not precipitate the colour¬ 
ing matter of the cochineal. 
I grind to a fine powder the samples to be tested, 
weigh out two or two and one-half grammes, and boil 
this amount in a capacious narrow-necked flask, with 
750 c. c. of water, for one hour. The liquid is imme¬ 
diately filtered through dry paper filters, and tested 
when cold. To test it, 50 c. c. are measured in a flask of 
that capacity and poured into another flask of about 
200 c. c., and the measuring vessel rinsed with a definite 
quantity of water, say 10-15 c. c. 
A weak solution of permanganate is then run in from 
a burette with a glass cock, the flask being shaken well 
after the addition of every 10 c. c. 
So much permanganate solution is added that the co¬ 
chineal extract shall be changed from its original colour 
to a pink of the very faintest shade, almost yellow, in 
fact, but never reaching a full yellow. This pink shade 
should be persistent, that is, it should not turn yellow 
after standing fifteen minutes ; and after a little practice 
it will be found very easy to obtain the tinge, which 
shows that the colouring matter is almost but not quite 
destroyed. 
When a number of samples are to be compared I 
arrange an equal number of 200 c. c. flasks and test-tubes 
on the table, a tube standing in its rack in front of each 
flask. Then the same number of c. c. of the perman¬ 
ganate solution (which should be at least so -weak that 
bulk for bulk of this and the cochineal solution will be 
required) is run into each flask, taking care to use too 
little to completely destroy the colouring matter in all . 
