INFLUENZA. Sce MEDICINE.
INK, a liquor used for printing or writing. The colour r.hiefly wanted for these purposes is black ; and the prin ipal properties of good ink arc, deepness of colour, dis tinctncss, and durableness. The vehicle employed for pro clueing adhesion of the colour to the substance on which it is impressed differs according to the instrument em ployed in forming the lines. It is chiefly on this accoun', that printing ink and writing ink are of so different com position. We shall describe the modes of making the best inks of these two kinds, and then the different coloured inks, and the compositions known by the name of sympa thetic inks, which last are, on the whole, more curiou than useful.
Printing ink is a mixture of black carbonaceous matter and oil, and makes a near approach to black paint. It re quires, however, some difference of mechanical proper tics. Paint is fixed by drawing a brush in lines along the substance to which it is applied. But printing ink is made to adhere by a pressure, without superficial motion. It re quires to be more tenacious and hard, but less tough and greasy than paint. This, and the other qualities, are com municated to it by a proper choice in the kind of oil, and by the preparation of boiling. Linseed and nut oil are the most suitable. The latter is the fittest for black ink ; but the boiling imparts to it a brownness, which injures the brightness of red ink. The other oils dry too slowly, and inks made of them come off and smear the paper, or the oils sink into its substance, so as to surround the letter with a yellow stain.
The oil is boiled in a pot large enough to hold at least half as much more, to prevent boiling over. While boil ing, it is constantly stirred with an iron ladle ; the surface also is kindled, and allowed to burn for half an hour, in order to consume and separate more completely the in flammable parts on which the greasiness chiefly depends. Alter the burning is extinguished, it is allowed to boil some time longer. A thick kind is made for use in hot weather, and a thinner for cold. Both go under the name of varnish. The thick varnish is known to be of the due consistence, when it draws into threads between the fin gers like weak glue. It is viscous, like a soft resinous juice. A proportion of turpentine or rosin is boiled along
with it, to give it body, and increase its drying quality ; some add a quantity of litharge. But as these admixtures render it extremely difficult to cleanse the types, it is much better to employ varnish which has acquired the same property merely by age.
The colouring matter generally employed is lamp black, two ounces and,a half to sixteen ounces of the varnish. They are ground together like paint. It is probable that the varnish has acquired by the preparation a gummy quality, and loses in part its oily constitution, a change which gives it the property of adhering to wetted paper. The substance left, when dried, is tough and flexible, and little disposed, either to mix again with oil, or with water. The gummy and oily matter were thought by Dr Lewis to be so proportioned, as to defend one another in some degree against the menstruum of each.
The preparing of printer's ink is a very delicate art, and hence is in the hands only of a few. The ink used in all parts of Great Britain is made in London. That which is used in France is somewhat different, and hence the French typography differs from the B itish. It has a bril liancy and distinctness which we cannot imitate, except by using the article prepared in France. Differences of opin ion exist as to whether these properties are advantages or not. Some complain of them, as accompanied with a daz zling effect.
Ink used for printing from engravings by the rolling press, must have less of the adhesive qualities, and a great er degree of yielding softness ; it must easily run into, and fill the hollow lines in the copper-plate, and at the same time be easily wiped off the polished sui face of the plate, previously to the taking of the impression. The peculiar ity in the manipulation of the varnish consists in giving it less boiling than that which is to be used for type ink. Lamp black is improper for engraver's ink, as it commu nicates to it an inconvenient toughness. The charcoal black ing, such as that called German or Frankfort black, sup posed to be procured from burnt vine-twigs, or kerne Is of fruits and wine lees, is in highest reputation, as more free from grittiness than the ivory black of this country.