upon incineration about 0.3 per cent. of ash,* and yields to acids perceptible traces of lime, magnesia, and sesquioxide of iron. It is always advisable, therefore, to treat even the Swedish filtering paper, when intended for use in minute and accurate investigations, with dilute hydrochloric acid, in order to free it from inorganic matter; after this process, the paper is carefully washed with water, to remove all traces of acid, and dried. In the case of very fine filtering paper, the best way to perform this operation, is to place the ready cut filters, either singly, or several together, in a funnel, exactly the same way as if intended for immediate filtration; they are then moistened with a mixture of one part of pure hydrochloric acid with two parts of water, which is allowed to act on them for about ten minutes; after this, all traces of the acid are carefully removed by washing the filters in the funnel repeatedly with warm water. The funnel being then covered with a piece of paper, turned over the edges, is put in a warm place until the filters are dry.

Ready-cut filters of various sizes should always be kept on hand. Filters are either cut by circular patterns of pasteboard or tin, or by Mohr's filter-pattern (Fig. 46), which consists of a quadrant made of

B

A

Fig. 46.

tin plate, A, the straight sides of which have a rim turned up to the height of about a quarter of an inch; the papers, properly folded, with the edges at a right angle, are put into this, and a flat piece of tin-plate of the same form, B, is placed over them. The outer edge of the paper is then cut, with the scissors, to the pattern.

Filters cut in this way are perfectly circular and of equal size.

Several pairs of tin plates of various sizes (3, 4, 5, 6, 7, and 8 centimetre radius) should be procured for this purpose. The filter ought always to be sufficiently large to afford room for double the quantity of precipitate remaining on it after the filtration of the fluid.

The funnels used for filtration should be quite even, and not bulge at the sides; they should be inclined at the angle of 60°. Glass is the most suitable material for them.

The filter should never protrude beyond the funnel; the best way is to select filters the respective radii of which are one or two lines shorter than those of the funnels into which they are to be inserted.

The filter is firmly pressed into the funnel, to make the paper fit closely to the side of the latter; it is then moistened with water, which is allowed to pass through the neck, but must, on no account, be poured off by inverting the funnel.

The stands shown in Figs. 47 and 48 complete the apparatus for filtering.

The stand in Fig. 47 is more particularly adapted for the reception of larger funnels, and should therefore be made a little more solid than that in Fig. 48, which is intended for the reception of funnels of smaller size. The stands are made of solid hard wood. The arm holding the funnel *Plantamour found the ash of Swedish filtering paper to consist of 63-23 silicic acid, 12.83 lime, 6·21 magnesia, 2·94 alumina, and 13·92 sesquioxide of iron, in 100 parts.

or funnels must slide easily up and down; but the screw, when tightened, must retain it firmly in position. The holes intended for the reception of the funnels, must be cut sloping conically downwards, to keep the

funnels steadily in their place. The arm b (Fig. 47) serves for the reception of a washing-bottle.

These stands are very convenient, and may be readily moved about without interfering with the operation.

$ 46.

bb. RULES TO BE OBSERVED IN THE PROCESS OF FILTRATION.

In the case of curdy, flocculent, gelatinous, or crystalline precipitates there is no great danger of any particles of the precipitate passing through the pores of the filter. But in cases where we have to deal with fine pulverulent precipitates, it is generally necessary and always advisable, to let the precipitate subside, and then filter the supernatant liquid, before proceeding to place the precipitate upon the filter. Substances which have been precipitated hot, are most properly filtered before cooling (provided always there be no objection to this course), since hot fluids run through the filter more speedily than cold ones. Some precipitates have a tendency to be carried through the pores of the filter along with the fluid; this may be prevented in some instances by modifying the latter. Thus sulphate of baryta, when filtered from an aqueous solution, passes rather easily through the filter-the addition of hydrochloric acid or chloride of ammonium prevents this in a great measure.

If the operator finds, during the filtration of a precipitate, that he has taken too small a filter to hold the precipitate conveniently, and so as to admit of its being properly washed, he had better use an additional filter, and thus distribute the precipitate over the two.

The fluid ought never to be poured directly upon the filter, but always

§ 52.

FIRST METHOD. (Heating the Precipitate to Redness, with simultaneous Incineration of the Filter.)

This method is resorted to in cases where there is no danger of a reduction of the precipitate by the action of the carbon of the filter. mode of proceeding is as follows::

The

The perfectly dry filter, with the precipitate, is removed from the funnel, and its sides are gathered together at the top, so that the precipitate lies inclosed as in a small bag. The filter is now put into the crucible, which is then covered and exposed, over a spirit-lamp with double draught or over gas, to a moderate heat, to effect the slow charring of the filter; the cover is now removed, the crucible placed obliquely over the flame, and a stronger degree of heat applied, until complete incineration of the filter is effected; the lid, which had in the meantime best be kept on a porcelain plate, or in a porcelain crucible, is put on again, and a red heat applied for some time longer, if needed; the crucible is now allowed to cool a little, and is then, while still hot, though no longer red hot,* taken off with a pair of tongs of brass or polished iron (Figs. 58 and 59), and put under the bell-glass, where it is left to cool; it is finally weighed.

X X E

Fig. 58.

Fig. 59.

Fig. 60. The combustion of the carbon of the filter may be promoted, in cases where it proceeds too slowly, by pushing the non-consumed particles, with a smooth and rather stout platinum wire, within the focus of the strongest action of the heat and air. Or the operator may also increase the draught of air, by leaning the lid of the crucible against the latter in the manner illustrated in Fig. 60.

It will occasionally happen that particles of the carbon of the filter obstinately resist incineration. In such cases the operation may be promoted by putting a small lump of fused, dry nitrate of ammonia into the crucible, placing on the lid and applying a gentle heat at first,

* Taking hold of a red-hot platinum crucible with a pair of brass pincers, might cause the formation of black rings round it.

which is gradually increased. However, as this way of proceeding is apt to involve some loss of substance, its application should not be made a general rule.

In cases where the precipitate is easily detached from the filter, the preceding method is occasionally modified in this, that the precipitate is put into the crucible, and the filter, with the still adhering particles, folded loosely together, and laid over the precipitate. In other respects, the operation is conducted in the manner above described.

§ 53.

SECOND METHOD. (Heating the Precipitate to Redness, and incinerating the Filter separately.)

This method is resorted to in cases where a reduction of the precipitate from the action of the carbon of the filter is apprehended; and also where the ignited precipitate is required for further investigation, in which the ash of the filter might form a disturbing element. It may be employed also, instead of the first method, in all cases where the precipitate is easily detached from the filter. The mode of proceeding is as follows::

The crucible intended to receive the precipitate is placed upon a sheet of glazed paper; the perfectly dry filter with the precipitate is taken out of the funnel, and gently pressed together over the paper, to detach the precipitate from the filter; the precipitate is now placed in the crucible, and the particles still adhering to the filter are removed from it, as far as practicable, by gentle scraping and friction, and are then also transferred to the crucible. The filter is now spread open upon the sheet of glazed paper, and then folded in form of a little square box, enclosed on all sides by the parts turned up; any minute particles of the precipitate that may have dropt on the glazed paper are brushed into this little box, with the aid of a small feather; the box is closed again, rolled up, and one end of a sufficiently long platinum wire spirally wound round it. The platinum crucible with the precipitate being placed on or above a porcelain plate, the little scroll within the coil is lighted, and, during its combustion, held over the crucible in a position to make the falling particles of the precipitate or filter ash drop into it, or, at least, into the porcelain plate. In this way, and by occasionally holding the little scroll again in or near the flame, the incineration of the filter is readily and completely effected. When the operation is terminated, a slight tap will suffice to drop the ash and the remaining particles of the precipitate into the crucible, which is then covered, and the ignition completed as in § 52. Where it is intended to keep the ash separate from the precipitate, it is made to drop into the lid of the crucible, in which case it is better to ignite the crucible with the principal portion of the precipitate first. This method of incinerating the filter, devised by Bunsen, is preferable to the method formerly in use, in which the filter, freed, as far as practicable, from the precipitate, was burnt on the lid of the crucible, the operation being aided and promoted by gently pressing the still unconsumed particles with a smooth platinum wire, or platinum spatula, against the red-hot lid, to bring them into the most intimate contact with it. No matter which method of incineration is resorted to, the operation must always be conducted in a spot entirely protected from currents of air.

Certain precipitates suffer some essential modification in their properties, in their solubility, for instance, from the application of a red heat. In cases where a portion of a substance of the kind is required, after the weighing, for some other purpose with which the effects of a red heat would interfere, the two operations of drying and heating to redness may be combined in the following way :-The precipitate is collected on a filter dried at 212° F.; it is then also dried, at 212° F., and weighed. A portion of the dry precipitate is put into a tared crucible, and its exact weight ascertained; it is then exposed to a red heat, allowed to cool in the usual way, and weighed again; the diminution of weight which it has undergone is calculated on the whole amount of the precipitate.

$ 54.

5. ANALYSIS BY MEASURE (VOLUMETRICAL ANALYSIS).

The principle of volumetrical analysis has been explained already in the " Introduction," where we have seen how the quantity of protoxide of iron present in a fluid may be determined by means of a solution of permanganate of potassa of known strength. I will, however, adduce here a few more examples in illustration of this principle.

Suppose we have prepared a solution of chloride of sodium containing in 100 cubic centimetres 0.5846 grm. of the salt; with this solution we are enabled to precipitate exactly 1.0797 grm. of silver from a solution of that metal in nitric acid-the equivalent of chloride of sodium being 58 46, that of silver 107.97. Now, suppose we have an alloy before us, consisting of unknown quantities of silver and copper, and we want to determine the amount of silver contained in it in this case, we need simply weigh off 10797 grm. of the alloy, dissolve carefully in nitric acid, and add to the solution our solution of chloride of sodium, drop by drop, until the whole of the silver is thrown down, and an additional drop fails to produce a further precipitate. The amount of silver present may now be calculated from the amount of solution of chloride of sodium used. Thus supposing we have used 80 cub. cent. of our solution, the amount of silver present in the alloy is 80 per cent.; since, as 100 cub. cent. of the solution of chloride of sodium will throw down 1.0797 grm. of pure silver, it follows that every cub. cent. of the chloride of sodium solution corresponds to 1 per cent. of silver.

=

Another example. It is well known that iodine and sulphuretted hydrogen cannot exist together: whenever these two substances are brought in contact, decomposition immediately ensues, the hydrogen separating from the sulphur and combining with the iodine (I + HS = HI + S). Hydriodic acid exercises no action on starch paste, whereas the least trace of free iodine imparts to the latter substance a blue tint. Now, if we prepare an alcoholic solution containing in 100 cub. cent. 0.7463 grm. of iodine, we may with this decompose exactly 1 decigramme of sulphuretted hydrogen, for 17: 126-88:01:07463. Let us suppose, then, we have before us a fluid containing an unknown amount of sulphuretted hydrogen, which it is our intention to determine. We need simply add to it a little starch paste, and then, drop by drop, our solution of iodine, until a persistent blue coloration of the fluid indicates