Apparent Specific Gravity.-The calculation of the volume composition of a paper under the system discussed in the "C. B.S. Units" throws considerable light upon the exact relation between the weight of a paper and its volume. This relation is, of course, the apparent specific gravity of the paper since

In other words, the weight of 1 c.c. of a paper expresses its "apparent specific gravity," a definite function which is of the greatest importance.

Breaking Weight per Unit of Sectional Area. --Another property of paper which demands attention in the consideration of volume composition relates to the strength of paper.

Usually the strength of a paper is recorded in terms of the number of pounds required to fracture a strip of given width, though in Germany the convenient expression of "breaking length," i.e., the length of paper which, if suspended, would break of its own weight, is almost exclusively employed.

The authors of the pamphlet "The C.B.S. Units" have shown that the following intimate relation exists between the terms-breaking length (7) and apparent specific gravity (s).

1 x 8

=

Breaking weight per unit of sectional area.

This third expression is a property of the paper which has not been studied to any great extent. The breaking weight per unit of sectional area is a familiar expression for recording the strength of structural materials, such as brass, iron and wood; but it is usually determined by direct experiments in which a bar of definite sectional area is fractured. The same method can be applied to paper, and the breaking weight per unit of sectional area as found by direct experiment agrees with the figure obtained by finding the product of breaking length multiplied by apparent specific gravity.

These various relations have an important bearing upon the quality of paper.

The C.B.S. Units applied to a Special Case.

Samples taken.-The author, during a course of lectures given at the Dickinson Institute, Croxley, under the auspices of the Hertfordshire County Council, endeavoured to explain the nature of the volume composition of paper, and the possibilities which might arise from a further study of the physical qualities of paper, by using as an illustration samples of some given paper taken from a machine at certain stages during manufacture, so that the alterations in physical qualities could be properly measured and the influence of successive operations determined.

The samples taken were as follows:

No. 1. Paper from between the couch-rolls and press-rolls.
No. 2. Paper between press-rolls and drying cylinders.
No. 3. Rough machine-finished paper after leaving cylinders.
No. 4. Rough M.F. paper lightly glazed, not tub-sized.

heavily glazed, not tub-sized.

These samples fairly represented the various stages in the manufacture of the paper. The paper taken from the couch-rolls was allowed to dry out naturally between blotting paper. The paper taken from the press-rolls was similarly treated. The remaining samples were all taken as rough machinefinished paper and submitted to the processes referred to.

The special investigations made with this paper are interesting in view of the changes in physical condition brought about during manufacture, particularly those relating to bulk and weight.

These changes are set out in Table XXXIX., which shows the weight per ream and thickness per ream at the various stages. The figures are instructive because they show the comparative weights for equal bulk, and the extra number of sheets required to make up the maximum bulk, so that the influence of each operation is clearly shown and measured.

Observations made. The actual observations were those of

(1) Weight.-Weighed pieces, each having an area of 10,000 square milliRecorded mean weight.

metres.

(2) Thickness. Took an average of independent readings on 1, 5 and 10 pieces respectively.

(3) Ash. By usual methods.

(4) Gelatine. By usual methods.

The figures given in Table XXXIX. have been calculated from the observations made.

Variations in Bulk. The differences in the substance and thickness are certainly interesting. The fact of their existence is well known, though the actual publication of a specific example now appears probably for the first time; but care is necessary in deducing any hard and fast conclusions from these

experiments. They are the results of an investigation made on one paper, and any suggestions arising can properly be applied only to this case. If, however, the results and conclusions obtained are repeated in similar investigations, and the relations found with the various physical qualities are consistent, then it will be possible to draw some useful facts from the experimental evidence which might and should eventually guide the paper maker. It is necessary to emphasise the necessity of only establishing a law on the evidence of repeated experiment, and not on single cases, in which the danger of a mere coincidence is obvious enough.

The effect of the natural shrinkage of the wet paper obtained from the couchand press rolls is very marked. Under the contraction due to natural evaporation of the moisture, the weight per ream of the first two samples reaches a high figure. In the rough machine-finished paper the forces of evaporation, tension, and the pressure of felts against the hot cylinders tend to give practically the minimum figure.

The changes in the thickness of the paper are also interesting. From sample one to sample five the reduction is uniform, the thickness of the first being 5 inches and of the fifth 2.77 inches per ream. The percentage loss of bulk at each stage may be seen in Table XL.

TABLE XL. Showing Reduction in Thickness of Paper at Various Stages.

The effect of the plate-glazing process is almost entirely confined to the reduction of bulk, and consequent relative increase in weight. The paper after glazing shows but little increase in area, so that the result of pressure as far as the weight is concerned is of a simple character. The increase in area determined by a few isolated measurements did not exceed 1 per cent.

The greatest reduction in bulk occurs with the first pressure of the plateglazing rolls. The subsequent rolling merely improves the surface. Other important points in reference to plate glazing will arise later.

Volume Composition. The results expressed in the familiar term of actual weight and thickness become still more interesting when studied in relation to the volume composition. The exact influence of each stage in the manufacture of the finished sheet is thus recorded in a form which shows the effect upon each constituent of the paper, and this is well illustrated by the special case under consideration. The volume composition of the paper at each stage in manufacture is set out in Table XLI.

Apparent Specific Gravity and Breaking Length.--The investigation of the special paper was continued with the view of determining the changes in physical qualities other than that of volume composition merely.

Observations were recorded as to the strength of the paper and the alterations in strength brought about by the several operations in manufacture.

Breaking Length. The term "breaking length" is seldom used in England, paper-makers contenting themselves with expressing the strength in terms of the weight required to break a strip of given width. In this case it is

necessary to record the width as well as the breaking strain, but by calculating the length of paper which, if suspended, would break of its own weight, a figure is obtained which is independent of the width, more or less. For example:

A paper 20" by 30" weighs 72 lbs. per ream, and has an average strength of 60 lbs., tested on a strip 1 inch wide. The weight of the strip. tested, if 5 inches long and 1 inch wide, should be th oz., as follows:

If the breaking strain of the paper is 60 lbs. for a one-inch strip, then a weight of 60 lbs. suspended to the strip 5 inches long, when hung vertically would break the paper. If instead of an actual weight, a length of paper

1 inch wide equal to 60 lbs. is substituted, then "breaking length" is determined.

Since th ounce is weight of strip 5 inches long

(250 × 16 × 60) inches, or 6666

The breaking length is therefore 6666 yards, irrespective more or less of the width.

Tests for strength. The strength of the special papers taken for investigation was determined in the usual way by taking six strips in the machine and cross directions, the dimensions of each strip being 1 inch wide, and about 5 inches long, giving a piece exactly 4 inches between the clamps of the apparatus. The observations made, and the calculations worked out, were as follows:

(1) Strength in the machine direction. Mean of six tests. (2) Strength in the cross direction. Mean of six tests.

(3) Average strength of the paper.

(4) Weight of a strip one metre in length, and 25 mm. wide. (5) Calculation of the "breaking length" of paper.

(6) Calculation of "breaking weight per unit of sectional area."