A Text-book of Paper-making. C. F. Cross
student.
Microscopical Examination.
—Under the head of “Microscopic Features” we must include everything which has to do with the structure of the vegetable fibres, as well as their organisation and distribution in the plant. In the analysis of “organised” structures we employ the two methods. (1) of dissection; (2) examination by means of the microscope; in other words, we first isolate the part under investigation by a mechanical process and then proceed to the optical resolution or analysis of the part. Having by analysis acquired a knowledge of the parts, we study their mutual relations in the structure they compose—we integrate our knowledge, so to speak—by means of sections of the structure, cut so as to preserve the cohesion of the parts in section, and yet in so fine a film as to appear under the microscope to be virtually a plane surface. These points are illustrated in the drawings given.
It is impossible for us to deal specially with the subject of the microscope and its manipulation. The microscope, as a revealer of natural wonders is one thing; as an instrument of scientific discovery, quite another. For the latter, the student must train himself by systematic work, and should especially concentrate his attention upon some one branch of natural history, however restricted.
We shall assume, in our treatment of the subject, a knowledge of the microscope as an instrument of research, such as can be easily acquired in a few weeks of work under the guidance of a teacher or of one of the excellent manuals which now abound. We also assume a certain acquaintance with the elements of vegetable physiology, which it will be seen is necessary for a full grasp of the subject. Such an acquaintance, also, may be easily acquired, under direction, in a few weeks of work.
FIG. 2.
FIG. 3.
We have before alluded to the differences presented by {33} mono and dicotyledonous stems in regard to the distribution of their fibrous constituents. In illustration of this, we may cite Figs. 2, 3, which represent, (2) a section of the aloe, (3) a section of the jute plant. The available fibres are in (2) the fibro-vascular bundles (f), which are irregularly distributed throughout the main mass of cellular tissue, and {34} in (3) the bast fibres (f), which constitute a definite and separate tissue. We have already alluded to the practical consequences of this typical difference of distribution, in regard to processes of separating these fibres on the large scale.
This process we have explained is necessarily simpler in the case of a fibrous tissue, definitely localised; and this may be demonstrated by a superficial examination of a young branch of an exogen. As we know, the bark tissues are easily stripped from the underlying wood. If now we work up the former in a mortar, with a little water, we soon perceive the separation of the compound tissue into cellular matter on the one hand and fibres, the latter being more or less long and silky, according to the plant from which isolated. They vary in length from one millimetre to several centimetres, and are aggregated together in the plant in such a way as to constitute bundles, often of very considerable length; the general arrangement being comparable with that of the tiles in the roof of a house. It is important to distinguish the fibre-bundles from the elementary or normal fibres, and to this end they are designated by the term filament. Bast fibres are flexible and fusiform, terminating gradually in a point at either end, as represented in Fig. 4; bast filaments, built up of these fibres, containing often as many as twelve in the bundle, are usually cylindrical, but exhibit the widest differences in regard to the aggregation, in degree as well as number of their constituents. It is obvious that while the spinner has to do with these filaments, the paper maker works up the ultimate fibre constituents or fibres. It is also an obvious corollary from this distinction that a fibrous material which from “weakness” is unavailable for textile application, may yet be perfectly “strong” from the paper maker’s point of view; in other words, the individual fibres may be strong, but have little cohesion in the filaments. As we proceed, the student will see more and more the practical bearings of this branch of the study, and will perceive the inferences to be drawn from the investigation of {35} minute relationships to manufacturing processes and their products.
FIG. 4.
We shall say but little as to the necessary equipment. (1) A dissecting microscope, for dissecting under a lens, magnifying the object to 40 or 50 diameters. (2) An ordinary student’s microscope with lenses for magnifying to 100 and 300 diameters. This is adequate to the work, though of course, it may be an advantage in certain cases, to be provided with higher powers. (3) A glass slide, carrying an engraved scale of centimetres and millimetres for measuring the lengths of objects, and a micrometer, divided into 1⁄10 mm. for measuring diameters. It is also important to be able to determine the degree of enlargement under any particular combination of lenses, and for this purpose to possess a micrometer eye-piece, with a millimetre scale divided into hundredths. (4) An effective microtome and the usual mounting accessories.
A very important feature in the diagnosis of fibres, more especially in regard to the composition of the fibre substances, is the effect produced by treatment with various reagents. Certain of these reactions we have already {36} indicated. We shall now give the details of composition of the several solutions which will be required.
Neutral Mounting Solutions.
—It is advantageous in mounting transparent objects to employ a solution of the same refractive power as the substance itself. For this purpose, pure glycerin or a syrupy solution of calcium chloride may be employed; it is expedient to mix with either reagent a small proportion of acetic acid. The designation “neutral” has reference to the fact that these reagents are without sensible action on the fibres.
Iodine Solution.
—We have previously described the preparation of a solution, giving the characteristic blue reaction with cellulose directly. It is, however, often preferable to bring about this colouration in another way, and the following are the solutions employed:—1 gramme of potassium iodide is dissolved in 100 cc. water, and the solution is saturated with iodine; it is preserved in stoppered bottles, containing a few fragments of the element, so as to keep up the saturation of the solution.
The accessory solution, dilute sulphuric acid, which is employed to determine the reaction between the cellulose and the iodine, is prepared as follows:—2 volumes of concentrated glycerin are mixed with 1 volume water, and to the mixture an equal volume of oil of vitriol (1·78 sp. gr.), is slowly added, so as to prevent as far as possible a rise of temperature. The effect of the glycerin is very remarkable in preventing the distortion of the objects under the action of the acid, which in other respects remains uninfluenced.
By way of verification of this iodine test, which is somewhat capricious, it is advisable to test the reagents with a standard substance. The best for the purpose is a linen yarn which has been partially bleached. Under the action of the reagents the fibres composing this yarn, which must, of course, be suitably “teazed out” for mounting, are coloured a light blue, the centre, however, showing a yellow line, {37} marking the distribution in the interior canal of a non-cellulose fibre constituent. Should these effects not appear, it may be concluded that the acid requires to be strengthened. On the other hand, too great a concentration is equally to be avoided; it is evidenced by causing a distortion of the fibre, easily recognised by comparison with the fibre mounted in a neutral medium.
Chlorine Water.
—One of the most characteristic reactions of