Now the points to be noted in connection with the blots are:

(1) The time required to produce a blot under the conditions set out.
(2) The total area or size of the blot.

(3) The proportion of the area occupied by the outer zone.

Some experimental results are embodied in the following tables:

TABLE XIX., showing Time required to produce a Blot from 05 cc. Ink.

TABLE XX., showing Total Area of Blots and Areas of Zones (square inches).

TABLE XXI.--Absorption Height in 10 minutes, in millimetres.

It is well known that the formation of a large outer zone indicates a poor quality of paper. If lines are carefully ruled across the blots in ink the nature of the outer zone is at once revealed, by the non-absorbing character of the surface of the paper which is covered by the outer zone.

Some curious results are obtained by producing a blot of ink in situ on the paper by means of tannic acid and ferric chloride. A blot is first formed on the paper by means of a 10 per cent. solution of ferric chloride, half a cubic centimetre being used as before. The yellow stain produced is allowed to dry, and half a cubic centimetre of tannic acid is allowed to drop on to the centre of the

yellow stain, the paper being turned over before the application of the tannic acid. A black ink mark is obtained owing to the combination of the two chemicals. The results obtained are very striking, and should be capable of

Fig. 109a.-Effect of producing a Blot by means of Iron Chloride and Tannic

Fig. 1097.-Effect of producing a Blot by means of Iron Chloride and Tannic
Acid in a Paper of Poor Quality.

Filter Papers.-Professor Herzberg has devised a simple apparatus for testing filter papers.

It consists of a long glass tube, the bottom of which is connected to a metal U-shaped tube provided with a tap. The sheet of the paper to be tested is inserted between the open end of the

U-shaped piece, and a small cap which can be fastened above the sheet of paper. The working of the apparatus is self-evident from the diagram. Water is maintained in the tube at a constant level, and the water which flows through the paper is caught in a measuring flask. For filter papers a rapid flow is desirable. For waterproof papers the cap can be modified in shape so as to form a simple clamp for holding the paper in position.

Imperviousness to Water and Air. The capacity of a paper for resisting moisture, as distinct from the extent to which it is sized is a quality peculiar to special papers, such as waterproof and grease-proof papers.

An approximate method for testing the water resistant properties of

B

Fig. 111.-Apparatus for Testing Airproof and Waterproof Condition of Paper.

Fig. 110.-Herzberg's Filter Paper
Testing Apparatus.

a paper is to immerse a sample of the
paper in the water for a few moments,
and then to weigh the paper after re-
moving very carefully all excess moisture
from the surface by means of blotting

paper.

A more accurate method is to fasten a piece of the paper securely to the end of a long glass tube. The tube is then filled with water to any desired height, and the effect of the pressure of the water on the sheet of paper which forms the bottom of the tube carefully noted. A poor paper will allow the moisture to pass through, so that the under surface of the paper becomes moist.

An apparatus suitable for testing how far a paper is impervious to air is shown in Fig. 111. It consists of a hollow glass tube A fitted at the upper end

with a suitable clamping arrangement, by means of which a sheet of the waterproof or grease-proof paper can be firmly fastened down at the upper end of the glass tube. The tube is then lowered into another vessel B containing water. The compression of the air inside the tube A, caused by immersing the lower end into the water contained in B, exerts a pressure on the sheet of paper. At the commencement of the test, the height at which the water stands in the tube A is carefully noted. If the paper is not impervious to air, the water will gradually rise to a higher level in the tube A, owing to the escape of air through the pores of the paper.

The "blister" test is sometimes used for grease-proof papers. The blister caused by a lighted match brought into contact with the under surface of the paper is due to the formation of steam in the paper from the natural moisture

I

Fig. 112.-Apparatus for the Measurement of the
Transparency of Paper.

always present in it. The outer surface of the paper being presumably waterproof does not allow the steam to escape rapidly. Turpentine oil can also be used with advantage. Grease-proof paper of good quality will not allow turpentine to pass through, and the additional advantage of the use of turpentine is that small holes which sometimes occur in parchment paper are readily detected, whilst the blister test does not show them.

The Transparency of Paper. The transparency of a paper may be expressed in terms of the number of thicknesses of paper which must be placed between the

eye of the observer and a standard light of known power in order to completely intercept the light.

Dr. Klemm has devised a simple apparatus for the accurate measurement of transparency, which consists of an illuminating tube and an observation tube. Between the two tubes is mounted the arrangement for holding the sheets to be tested, the number of which is increased until they completely intercept the light. This number, multiplied by the weight per square metre, gives the relative transparency. The luminous field is furnished by a Hefner-Alteneck amyl-acetate lamp of one-candle power mounted at a distance of 10 centimetres from the end of the ground glass disc which forms the inner cover plate. The distance of the observation tube (the normal length of which is 25 centimetres) is regulated to suit the observer's vision by means of a rack and pinion. The cover plate of the inner end of this tube consists of a square frame provided with hooks to hold the sheets as they are placed in the frame. As each fresh sheet is inserted, the observation tube is screwed back till it touches the glass plate of the illuminating tube so as to hold the sheets in a vertical position. Stray light is avoided by a cap covering the illuminating tube.

The number of sheets required to render the light invisible is a measure of its transparency. If, for example, twenty sheets of paper are required to obliterate

the light, the paper will be one-twentieth of the thickness necessary to render it opaque. This fraction is a measure of the absolute transparency of the

paper.

By taking the weight of the paper into consideration, then the relative value of the transparency of the paper can be expressed in simple numerical terms. Example. Let number of sheets used to obliterate the light be N. Let W equal the weight of a given area of paper.

Then relative value of transparency is WN.

Colour and General Appearance. These properties of a paper are not usually expressed in numerical terms, as it is exceedingly difficult to find methods. of recording them. The only

means for registering the colour of paper is by the use of an instrument known as the Tintometer. This instrument is employed in many industries, such as the manufacture of sugar, paint, oils, soaps, iron, steel, flour, &c.

The optical part of the apparatus B consists of a double parallel-sided tube of square section, fitted with plain holes. for binocular vision C, and with grooved partitions at the lower end J. The instrument can be tilted to any required angle. (Fig. 113.)

The opaque substance to be measured is placed on the foot of the base F, opposite one of the tubes, and a standard white surface is placed by its side opposite the other tube. Standard coloured glasses are then inserted in the grooves of the tube opposite the standard white surface until the colour of the latter, viewed through the apparatus, matches the unknown or unmeasured colour.

1

1

The standard white surface is prepared in the following way: "A represents two pieces of glass, one larger than the other, cemented together, the smaller piece just fitting into B, which is a rectangular frame of an inch deep; this fits to a tray C, also of an inch deep. B and C are fitted together, filled with the standard white substance, and the latter is then levelled off; the powder is pressed lightly down with the small side of the glass presser, in order to remove the frame B more easily from the tray C. The broad side of the presser is used to compress the powder even with the sides of the tray. The whole arrangement is shown in section at D." (Fig. 114.)

The tray is placed at F, opposite one tube, and the paper to be matched previously pinned on to a small block of wood equal in size and height to the tray, opposite the second tube.

The insertion of coloured glasses is an operation which is readily mastered after a little practice and assistance from some one accustomed to the instrument.

The standard glasses.-A complete series of glasses consists of 155 slips for red, yellow, blue; making 465 in all, but a full set is only required when very marked differences in colour are being measured. The gradation in each colour is the same, commencing with 01 (as the lowest practical unit, although others lower