precipitate, then silicates of alumina (indicating china-clay) and of magnesium (indicating agalite, asbestine, &c.) may be present.

The silica is carefully filtered off, and the clear filtered liquor examined for alumina and magnesia.

A portion of the solution is heated in a beaker, and ammonia cautiously added until the liquid retains a permanent smell of ammonia. If no precipitate is produced, showing the absence of alumina, then the original ash contained no china-clay.

If a copious gelatinous precipitate is obtained alumina is present, and indicates that the ash contained china-clay.

The solution, if no precipitate is produced, is cooled and a weak solution of sodium phosphate added to it. A crystalline precipitate obtained indicates magnesium, and this may be taken as a proof of the presence of one of the silicates of magnesia.

The solution, if a precipitate is produced, is boiled for ten minutes in order that all the alumina may be completely separated out. It is then filtered and examined for magnesium as described. Absence of the characteristic crystalline precipitate when sodium phosphate is added indicates the absence of any silicates of magnesia in the ash of the paper.

(B) The ash is coloured.

If the ash derived from the burning of a paper is coloured, some indication of the pigment used as the colouring medium may be obtained. Aniline or vegetable colours are destroyed by the combustion of the paper, and these must be examined by another process.

Blue.-If the ash is blue, ultramarine, prussian blue, or smalts may be present. Smalts is seldom used on account of its high price.

The colour of ultramarine is discharged by dilute acids, but is unacted on by alkalies. The colour of prussian blue is discharged by alkalies, but is not altered by weak acids. The colour of smalts is not affected by acids or alkalies.

A mixture of prussian blue and ultramarine may be tested by boiling with some caustic soda and filtering. The ultramarine remains on the filter paper unchanged in colour, while the prussian blue passes into solution. A few drops of ferric chloride added to the filtrate, which is first made acid with hydrochloric acid, will re-form the prussian blue as a dark blue precipitate.

Yellow. If the ash is pure yellow, it suggests chrome yellow (canary yellow) as the pigment. The pigment is readily soluble in weak caustic soda solution, which becomes yellow in colour.

If the ash is reddish yellow, of varying shades, the pigment may be orange mineral, or one of the many light-coloured ochres.

Red.-If the ash is red, some pigment which owes its colour to oxides of iron is present. Such pigments are venetian red, which dissolves when boiled in strong hydrochloric acid; indian red, burnt sienna, red ochre, which are more or less soluble in a mixture of hydrochloric and nitric acids.

Green.-A greenish coloured ash may be due to the use of a mixture of blue ultramarine with canary yellow. The ash, if treated with weak acid, loses all its colour; if treated with caustic soda, the yellow pigment is dissolved and the blue ultramarine remains unaltered and can be separated by filtration.

Buff. This colour is usually due to the addition of nitrate of iron or to some pale coloured ochre.

Brown.-A brown ash may indicate manganese-brown. On treatment with dilute hydrochloric acid, a rose-coloured solution is obtained.

Estimation of Ash.-The mineral residue which remains when a known

weight of paper is burnt is usually called "ash."

It may vary from 0.02 per cent., as in the case of pure filter papers, to 35 per

cent., which occurs in coated papers, the normal conditions being approximately those shown in Table XXIII.

The ash found in papers must not be attributed invariably to what is termed "loading," that is, mineral matter directly added to pulp. In many cases there may be 2 to 3 per cent. of ash present arising from causes of an entirely different

character.

In the first place, the fibre always contains a small percentage of natural ash, the amount varying according to the particular class of fibre. The mineral residue in ordinary paper-making materials as given by Herzberg is:

In the second place the chemicals used for sizing and colouring paper give mineral residues also, since the alum, ultramarine, and pigments of various kinds

Fig. 118.-Chemical Balance.

Example.

produce ash.

The amount of ash in paper is readily ascertained by burning a carefully weighed quantity of paper and weighing the residue.

For this purpose about one gramme of the paper is accurately weighed on a delicate balance, placed in a porcelain crucible and burnt by means of a gas burner or a spirit lamp until all the carbonaceous matter has been driven off. The crucible is allowed to cool and then weighed with its contents, the weight of the crucible when empty having been previously determined.

If the paper contains only a small proportion of residue, it is better to take two or three grammes, unless the balance is sufficiently sensitive for weighing the traces of ash left from the ignition of one gramme of paper.

Several appliances have been constructed for determining the ash in paper by means of which it is possible to dispense with the ordinary chemical balance and weights, but these are not very largely employed.

The ash balance of Post.-This consists of a delicately suspended lever, to one end of which is fixed a long pointer, and at the other end is fastened a small wire frame or scalepan. The pointer indicates on a scale, divided from 0 to 150,

Fig. 119.-Post's Ash Balance.

Fig. 120.-Crucible containing Paper, with Bunsen Burner ready for use.

any increase in weight equal to one-hundredth part of a gramme for each division. The balance is used in the following manner: The special platinum wire net provided with the apparatus is placed on the scalepan fixed at one end of the lever, and the instrument adjusted to zero, by means of levelling screws. The paper to be tested is twisted and placed in the wire net, and its weight noted as recorded in divisions on the scale. The wire net with its contents is then submitted to the flame of a Bunsen burner, or a spirit lamp, until the paper has been completely burnt. When the net is cold, it is placed on the balance and the number of divisions on the scale again read off. The reading indicates the relative weight of ash, and from this the percentage of ash can be calculated.

It is sometimes convenient to take exactly one gramme of paper, giving 100 divisions on the scale, so that the second reading indicates at once the percentage of ash without calculation.

When frequent deterininations of ash in paper are necessary, it is important and often essential to minimise as far as possible all labour involved.

In the paper-mill, for example, the percentage of ash must be determined by the chemist within a few minutes, and every means is devised for this purpose. Usually the rapid and continual testing of papers for ash is a question of proper

manipulation of simple appliances, and not the employment of elaborate and expensive apparatus. A fairly sensitive chemical balance, two or three porcelain crucibles, a spirit lamp or Bunsen burner, an iron tripod, pipe-clay triangles and crucible tongs form a complete outfit. While a specially contrived ash balance and combustion apparatus have certain advantages, the ordinary stock appliances are really more useful, because they can be employed for many other purposes.

Fig. 121.--Apparatus for burning Paper for Ash Determination

by means of an Electric Current.

Moisture in Paper.-As the amount of natural moisture in paper varies according to atmospheric conditions, it is sometimes necessary to determine the exact percentage.

One or two grammes of paper are weighed out exactly and dried at 212° Fahr. in a suitable air bath. The paper, while still hot, is placed in a small weighing bottle which is then placed in a desiccator till cool. The bottle is weighed with its contents, and again weighed when empty, the difference in the two weighings giving the exact weight of the bone-dry paper. From these figures the moisture can be calculated.

Example.-Paper taken, 1:50 grammes.

14.3000

1.3500 97

10.0 per cent.

Except under special circumstances a knowledge of the exact variation of the percentage of moisture in paper, due to altered atmospheric conditions, is seldom required. The alterations of the weight of the paper and of the humidity of the air can be observed simultaneously by weighing the paper at intervals during free exposure to the air, and noting the hygrometric condition of the atmosphere by means of a dry and wet bulb hygrometer.

Fig. 122.-Drying Oven.

This instrument consists of two thermometers suitably mounted on a stand, one having its bulb exposed to air in the ordinary way, the other having the bulb kept continually moist by means of a cotton wick dipping into a small reservoir of water. When the air is very dry water evaporates quickly from the surface of the wet bulb, which consequently is much cooler than the dry bulb. When the air is damp, evaporation takes place slowly, and the wet bulb thermometer does not indicate a much lower reading than the dry bulb thermometer. (Fig. 123.) The difference in the readings of the two thermometers is a measure of air moisture, and by reference to special tables the relative humidity of the air, expressed in terms of the percentage of moisture actually present compared with that required to completely saturate it, may be found.

The paper can be suspended in any convenient position in a room free from artificial heat, and weighed as required. If a large sheet is hung by some special contrivance to the arm of a Leunig paper-scales permanently for the whole period of investigation, observations can be made conveniently without any trouble.

Starch. The presence of starch in paper is readily detected by means of a dilute solution of iodine in potassium iodide. A few drops of the iodine test reagent used for microscopic analysis are added to distilled water in quantity sufficient to give a yellow liquid. A blue coloration is almost immediately imparted to papers containing starch, when these are dipped into the liquid. With 4 per cent. of starch, the colour is intensely blue, almost black. Careful observa