The sizing of coated papers should be neutral, but is frequently alkaline or acid, since alkali is used to neutralize the lactic acid of the casein. This may be detected by taste. The retention of a piece of coated paper in the mouth for a few minutes will reveal through the taste any tendency of the coating to sour. WEIGHT AND BULK.--Weight and bulk may be closely approximated by a practiced hand, but they must also be considered in relation to finish, as pointed out in the preceding chapter. There are many convenient forms of micrometer gauges for measuring the thickness of paper and any one who has much to do with paper should be provided with one, as it is unsafe to depend entirely upon judgment when a thousandth part of an inch may account for ten pounds difference in the weight of a ream of paper or cause serious variations in the bulk of a book. QUALITY AND STRENGTH.--Quality and strength may be approximately judged by tearing the paper in both directions of the grain and observing the fractured fibers, but these matters are to be more accurately estimated by mechanical and chemical tests. It will be observed that cleanliness in paper, and most of the foregoing characteristics of paper, do not lend themselves to mechanical tests, but are properties which require the judgment of an expert. CARDBOARDS.--In judging thick papers, such as bristol boards, it is customary to see if they are snappy. An idea of their fibrous strength may be had by folding in various directions. Pasted cardboards may be distinguished from unpasted by burning, for if paste has been used the layers of paper will split apart as the paper burns. This burning will also give a slight idea of the amount of filler in the stock, as the ash will be greater as the filler is increased. PAPER-TESTING.--Tests applicable to paper may be divided into three classes--microscopical, physical and chemical. The purpose of microscopical tests is to determine the kind and character of the fibers, and the proportion of each kind, also to assist in determining the nature of mineral filler and of impurities. It is also used in estimating the percentages of the various kinds of fiber. Chemists are able to estimate this within five per cent. A minute sample of paper is prepared by boiling in a one per cent solution of sodium hydroxid, in order to remove everything from the fibers themselves. The resulting mite of pulp is placed on a slide with a dissecting needle, the excess moisture is removed and a stain is added. This stain gives different characteristic hues to the different kinds of fibers. The color and form of the fibers as observed through the microscope disclose their character to the trained eye. By counting the different kinds of fibers under observation, the analyst estimates the proportions in which they existed in the sample of paper.