Pectic acid, though it rarely exists ready-formed in the plant, can readily be obtained from it by the action of weak alkalis. It is best procured by boiling the pulp of turnips with a 10 per cent. solution of sodium carbonate for about half an hour. If an acid be added to the filtered liquid, pectic acid is precipitated as a transparent colourless jelly, which {29} dries up to a transparent horny mass. By the prolonged action of alkalis or acids, pectic acid is converted into metapectic acid (C8H14O9), which may be obtained as a syrupy, strongly acid liquid.
From what we know of the properties of the members of the pectic group, and therefore of the pecto-celluloses, it will be readily seen that the latter are very liable under the action of alkalis to undergo conversion into soluble derivatives.
Conversely, a study of the action of alkalis upon plant substances, enables us to determine to what extent the latter partake of the nature of the pecto-cellulose, the importance of which to the practical paper maker we need hardly point out. For a further development of this idea (see p. [43]).
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CHAPTER II.
PHYSICAL STRUCTURE OF FIBRES.
We now have to treat of the fibrous raw materials from the point of view of form or structure, which is, of course, a very important factor in determining the quality of the paper or other fabric into which they are manufactured. It is sufficiently evident that the strength of paper is primarily due to the cohesion of its constituent fibres. When paper is torn, the edges present a fibrous appearance, and observation teaches us that, other things being equal, the greater the manifestation of fibrous structure the stronger will be the paper. If a paper be thoroughly wetted, its tensile strength is reduced to a minimum, and if subjected to a slight strain we get, not a tearing, but a pulling asunder of the fibres. If this be performed under a lens, the structure of the paper is more clearly seen, and it will be appreciated to what extent the qualities of a paper are the aggregate of the qualities of its constituent fibres. A more careful dissection of the paper shows that these fibres, which are the ultimate fibres of the plant, as distinguished from the bundles of these, or filaments, which the spinner employs, are interlaced in all directions. To produce this effect of interweaving, and to insure that uniformity which is an essential feature of good paper, we have among others the following contributory causes: (1) the deposition of the fibres from suspension in water; (2) the composition of the pulp with regard to the reduction of the fibrous bundles, and the isolation of the individual fibres; and (3) the structure of these ultimate fibres. It is with the last that we are chiefly concerned at present.
To convey a general notion of the influence of the structure {31} of fibres upon fabrics, we shall with advantage travel outside our immediate province to consider briefly the woollen and silk manufactures in relation to this point. Wool is, as we know, a discontinuous fibre, and its structure is that represented in Fig. 1, the most conspicuous feature being its broken surface, consisting apparently of imbricated scales.
FIG. 1.