1. A small strip (a square inch, for instance) of the suspected cloth is immersed for 2 or 3 minutes in a boiling mixture of about equal parts of hydrate of potassium and water, contained in a vessel of silver, porcelain, or hard glass; after which it is taken out and pressed between the folds of white blotting paper or porous calico. By separating 8 or 10 threads in each direction, their colour may
be readily seen. The deep yellow threads are LINEN, the white or pale yellow ones are COTTON.
2. A small strip of the cloth, after having been repeatedly washed with rain water, boiled in the water, and dried, is immersed for 1 to 2 minutes in sulphuric acid; it is then withdrawn, carefully pressed under water with the fingers, washed, immersed for a few seconds in ammonia, solution of carbonate of potassium, or solution of carbonate of sodium, again washed with water, and dried between filtering paper. By this treatment the cotton fibres are dissolved, while the linen fibres are merely rendered thinner and more translucent according to the duration of the experiment; after a short immersion, the cotton fibres appear transparent, while the linen fibres remain white and opaque.
3. Böttger recommends the linen stuffs to be dipped into an alcoholic solution of rosolic acid, then into a concentration solution of sodium carbonate, and finally washed with water. The linen fibre assumes a pink colour, whilst the cotton fibre remains unaltered.
4. (By the MICROSCOPE.) The indications afforded by both the previous tests, although quite visible to the naked eye, are rendered still more palpable by the use of a magnifying glass of small power, as the common pocket lens. Under a good microscope the presence of cotton in a linen tissue is very perceptible. The fibres of cotton present a distinctly flat and shrivelled appearance, not unlike that of a narrow, twisted ribbon, with only occasional joints; whilst those of flax are round, straight, and jointed. The fibres of cotton, after being exposed to the action of strong alkaline lyes, untwist themselves, contract in length, and assume a rounded form, but still continue distinct in appearance from the fibres of linen. The engraving represents a fibre of linen (1) and a fibre of cotton (2), as they appear when magnified 155 diameters. The difference between the two may be perceived, although less distinctly through a good Stanhope or Coddington lens, provided the object be well illuminated.
Dyeing. Linen and cotton, from the similarity of their behaviour with dye-stuffs, are treated in nearly the same manner. The affinity of their fibres for colouring matter is very much weaker than that of the fibres of silk and woollen. On this account they are dyed with greater difficulty than those substances, and the colours so imparted are, in general, less brilliant and permanent under similar conditions. Linen shows less disposition to take dyes than cotton. The yarn or
cloth, after being scoured and bleached in the usual manner, requires to have an additional tendency given to it, by chemical means, to condense and retain the materials of the dye-bath in its pores. This is effected by steeping the goods in solutions (mordants) which have at once an affinity for both the fibres of the cloth and the colouring matter. A similar process is employed in dyeing most other substances; but with cotton and linen, attention to this point is essential to the permanency of the dye. These matters are more fully explained under the heads Dyeing and Mordant.
The following new process for bleaching linen, having been omitted from our article on “Bleaching” is inserted here:—
Mr Hodges’ process, which is known in Ireland as the “Chemico-Mechanical Process,” owing to the patentee turning to account the advantages derivable from the employment of mechanical contrivances driven by steam, combined with the introduction of a new method of obtaining the hitherto little used hypochlorite of magnesia, may be said to date from the discovery of the substance, known as Kieserite (native sulphate of magnesia), which occurs as an essential constituent of the Abraumsalts of Stassfurth. For some time after the introduction of this substance into the market, it was considered of little value except for the production of Epsom salts; but Mr. Hodges, in the course of some investigations in bleaching jute, having had occasion to employ large quantities of hypochlorite of magnesia, it occurred to him that kieserite might be substituted for the more expensive crude sulphate of magnesia; and the importation into Ireland of the sample for this purpose was the first that was ever sent into that country for the manufacture of a bleaching liquor, or, indeed, for any other use. Mr. Hodges on experimenting with the kieserite, found that it not only supplied the place of the crude sulphate, but acted as a better precipitant for the lime of the bleaching powder, which is employed in the production of the hypochlorite of magnesia; and that it also produced a stronger and clearer solution. Without entering into a minute description of the process (which is at present successfully carried out in a factory erected for the purpose in the neighbourhood of Belfast), the following outline will be sufficient to show the nature of the methods adopted. The kieserite, which is imported from Germany in square blocks, on arriving at the works, is conveyed to a house, on the ground-floor of which it is stacked until required, when it is ground to a fine powder, and placed in barrels, is drawn up by means of a crane to a room at the top of the building, at one end of which is a row of three tanks furnished with water taps, agitators, and false bottoms. In one of the end tanks a definite quantity of the kieserite powder (varying according to its strength, is ascertained