(b) A weighed quantity of paper can be extracted with alcohol in a Soxhlet apparatus and the rosin determined by the loss of weight due to extraction.

(c) A known weight of paper extracted with alcohol by successive extractions as described above yields a solution containing a weighable quantity of resinous matter. This can be dried and weighed.

(d) A simple colour measurement, based on the turbidity produced by pouring the alcoholic extract into water, affords an approximate result. The test is made under the following conditions:

A standard solution of rosin in absolute alcohol is prepared by dissolving 1 gramme of rosin in 100 c.c. of alcohol. The extract from a known weight of paper is added to 50 c.c. distilled water in a colourless glass tube (a flat-bottomed Nessler tube is most suitable). To 50 c.c. of water in another Nessler tube sufficient standard rosin solution is added from a graduated pipette to give a turbidity equal to that produced from the paper. The amount of rosin is thus determined.

Example.-Paper taken, 20 grammes.

Volume of standard rosin used 3.0 c.c.
per cent. rosin in paper = 50 × 3·0 × ·01
= 1.5 per cent.

Soluble Matters in Paper.-The Soxhlet apparatus may be used for removing all soluble matters from paper, those soluble in water by means of water, and those soluble in alcohol by means of alcohol.

It is particularly convenient for treating large quantities of paper with small quantities of solvent in order to obtain a solution containing a maximum proportion of extracted matter. As shown in the diagram, the apparatus is fitted up for the extraction of paper with alcohol, to remove bodies soluble in alcohol, such as rosin and aniline dyes.

The paper, previously weighed and rolled up tightly into cylindrical form, is placed in the extracThe extractor is fitted into the neck of a boiling flask h, which rests upon a water bath k, heated by means of a Bunsen burner m.

tor a.

e

d

ub

h

m

k

Fig. 127.-Soxhlet Appa-
ratus for extracting
Soluble Matters from
Paper.

In the top of the extractor is fitted a condenser d. When the burner is lighted the water boils and heats the contents of the flask h. The alcohol boils, passes upwards into the extractor a through the side tube b and into the internal globe of the condenser. It is then condensed by contact with the sides of the condenser, kept cool by a current of water passing into the tube ƒ and from the tube g. The condensed alcohol falls back upon the paper in the extractor a, and when the vessel a is filled the alcohol syphons back automatically through the syphon tube c into the flask h, carrying with it more or less of the matter in the paper soluble in alcohol.

The process is continuous. After an hour or more, according to circumstances, the paper may be taken out and a fresh piece put in, without disturbing the solution in the flask. In this way a very small quantity of alcohol is sufficient for a large amount of paper. The paper can be dried, and the loss of weight ascertained.

The solution can be kept for subsequent examination.

The Abnormal Chemical Constituents of Paper.

The substances occasionally found in paper which are detrimental to its qualities of colour, durability and appearance may be most conveniently described as "abnormal constituents." They cannot be rigidly classified in the same manner as the normal constituents, nor can they be fully discussed, since they partake of the nature of impurities, some present by reason of careless manufacture, others on account of the use to which the paper has been put. The only adequate discussion of the subject, which is an interesting and important one, must arise from a consideration of specific cases, as suggested in chap. ii.

Free Acid in Paper.-The use of a solution of litmus is scarcely permissible for detecting acidity in papers. In ordinary cases a solution of neutral purple litmus is coloured red by the merest trace of free acid, but with paper litmus may be reddened even in the absence of free acid. This peculiarity is due to the presence of sulphate of alumina in small quantity. Alum is a salt which has what is known as an "acid reaction," and turns blue litmus red, so that this colour does not always indicate free acid.

The proper indicator for the detection of free acid is congo-red, a reddish colouring-matter which turns blue with traces of free acid.

The suspected paper is cut into strips, and extracted with very small quantities of distilled water in test-tubes by the method described for extracting rosin size from papers so as to produce a fairly concentrated extract. The solution is poured out into two or three watch glasses, and test-papers immersed in each, a blue litmus paper in one, a congo-red test-paper in the second.

If the litmus goes red, and the congo-red paper turns blue, the suspected paper contains free acid. If the litmus goes red, but the congo-red remains unchanged, then the acidity is only apparent, being due to the presence of the alum.

The test may also be made by putting one drop each of litmus and congo-red solutions on strips of the paper, allowing them to remain in contact for an hour or more with the strips while covered over with inverted watch glasses.

It is very desirable that papers intended for water-colour paintings, drawings, needle wrappers, steel and ironware wrappers, cover papers upon which gilt letters are to be blocked, and photographic papers, should be quite free from any suspicion of free acid.

Sulphur and Sulphur Compounds. The presence of sulphur and sulphur compounds in paper is very detrimental. The exceptions may be noted of the normal sulphates, such as barium sulphate, calcium sulphate, and alum, which are innocuous, and occur in most cases as normal constituents of paper.

Sulphur compounds of the undesirable type, which yield sulphuretted hydrogen when heated with dilute acids, are detected by boiling pieces of the paper with dilute hydrochloric acid, the steam being allowed to impinge on acetate of lead test-paper which turns brown when sulphuretted hydrogen comes into contact with it.

These compounds arise from a variety of causes. The use of hyposulphite of soda as an antichlor for killing bleach is apt to produce a precipitate of sulphur. Badly boiled and incompletely washed chemical wood pulp contains traces of sulphides from substances used in manufacture.

The objections to the presence of such impurities are to be found in the unpleasant smell of the paper, the odour being particularly noticeable when the reams of paper are first opened; also in the serious drawback of a brownish halo round the letters produced with ordinary printer's ink, caused by the action of the sulphur compounds on the oily constituents of the ink.

Free Chlorine and Chlorine Compounds. Free chlorine, as such, is seldom, if ever, found in finished paper. The paper-maker guards against the presence of any free chlorine in the bleached pulp by testing it with a solution of starch paste to which a little potassium iodide has been added.

Any free chlorine which may remain in the pulp is gradually changed into chlorides, and these may be found in paper as chlorides of sodium, calcium, magnesium, and sometimes iron.

The paper is extracted with distilled water acidified with a few drops of nitric acid known to be free from chlorides. To the extract is added a small quantity of silver nitrate, which will produce a more or less copious precipitate or merely a slight cloudiness, the latter being more frequently observed. The presence of much calcium chloride indicates, as a rule, an incomplete washing of the pulp after bleaching.

Mineral Adulterations. The addition of mineral matter to paper cannot be regarded as a process of adulteration, unless the presence of loading has been strictly forbidden by contract. The nature of the ash must then be determined by methods already described.

Particles of Metal, Dirt, &c.-The nature of the particles of metal and foreign matter in paper may largely be determined by careful microscopic analysis.

Iron and copper particles may be identified by dipping the paper into weak hydrochloric acid, removing the excess of acid by blotting paper, and then immersing the sheet of paper in a dilute solution of potassium ferrocyanide. Each particle of iron is surrounded by a blue-coloured circle, and the copper by a chocolate-coloured circle.

Particles of suspected coal or coke, when carefully picked out, will, if strongly heated on a piece of platinum foil, burn to a white ash,

CHAPTER XI

THE MICROSCOPE

Description of the instrument and accessories-General hints on the use of the microscope -Mounting, examination and identification of fibres-Exercises for studentsMeasuring the dimensions of fibres-Reagents for microscopical work

THE microscope is an indispensable instrument for those who have anything to do with the technical examination of papers. By means of it one is able to determine, not only the nature of the fibrous constituents in the paper, but also approximately the proportions in which those constituents are present. The identification of the fibres is a matter which requires considerable experience and constant practice, particularly with papers containing mixtures of fibres which closely resemble one another in physical structure.

The microscope can also be used for the examination of the mineral residues in paper, and for many investigations of an important character quite outside the routine work of ordinary identifications.

The Instrument.-For routine work an elaborate microscope is by no means necessary. The most convenient instrument for general practice is one which permits of a magnification ranging from about 60 diameters up to 300 diameters, the latter magnification being seldom required. The instrument should be provided with a coarse and a fine adjustment, two ordinary eye-pieces, one-inch objective and one 1-inch. objective; also a mechanical stage, which is most useful when examining papers for the determination of the proportions of different fibres present.

The writer recommends an instrument made by Messrs. Watson, of Holborn, known as "The Edinburgh Students' Microscope" (Stand H), provided with eye-pieces B and C, together with 1-inch and 1-inch objectives. With this combination the following range of enlargements is possible:

From the above table it will be seen that with the eye-piece and objectives mentioned it is possible to obtain a magnification of 72 diameters by using eyepiece B, 4-inch objective, with the tube of the instrument maintained at its

short length; and the magnification of 336 diameters by using eye-piece C, with -inch objective, and the tube of the microscope racked out to the full length of 10 inches.

Whatever type of instrument is selected, it is important that the stand should be perfectly rigid, particularly when the microscope is tilted into a horizontal position, the latter condition being necessary when it is desired to make drawings of the appearance of the fibres under the microscope.

General Hints on the Use of the Microscope.

It is a common fault on the part of many students who take up work with the microscope for the first time to imagine that it is best to employ high powers to carry out ordinary investigations. As a matter of fact, it is better to use low powers as far as possible, and to confine the use of higher powers to special investigations on the structure of

individual fibres.

Mr. Lewis Wright, a well-known authority on the microscope, further points out that the eye needs educating as well as the hands; that is to say, the expert manipulation of the microscope demands considerable practice not merely as to the use of the various parts of the microscope, but also as to the training of the eye to observe fundamental details, which might at first sight appear of small

moment.

As this chapter has been written mainly for the benefit of students who are not familiar with the microscope and its uses, the general hints as to its manipulation have been written in the form of instructions, to be carefully followed out.

The Instrument.-The base of the instrument called the foot, K, is usually tripod in form, which is the best shape for steady work. The body of the instrument can be tilted at any angle between a vertical position and a horizontal position by means of the pivot M, the exact horizontal position being insured by a small stop-bar H. The tube or body G carries at the upper end the eye-piece A, and at the lower end the objective D. It contains also a sliding draw-tube B, so that the length of the body can be varied from about six to ten inches. The body is lengthened merely by drawing out the slide-tube to any required distance. The regulation of the tube is obtained by means of the coarse adjustment, consisting of the rack work actuated by means of milled-head screws C. The fine adjustment F is only required when the higher objectives are used. As the name implies, it is used for bringing the object into exact focus by means of very small movements of the body.

The stage E, which is used for carrying the slide, in its simplest form is

K