from five to six centimeters in width. This last is covered by a disk
of unpolished glass. Into the bottom of this vessel introduce from
twenty to thirty grams of iodine in crystals.

Place the portion of paper on which the vapor of iodine is to act at
the opening of the bottle, and cover it with the stopper of unpolished
glass, on which put a weight so as to exert a slight pressure, and in
order that the aperture may be hermetically closed. Then allow the
vapor of iodine to act on the dry paper for three or four minutes at
the temperature of 15° to 16° C. and examine it attentively. When the
surface has not been spotted by any liquid (water, alcohol, salt
water, vinegar, saliva, tears, urine acids, acid salts, or alkalis) a
uniform pale-yellow or yellowish-brown tinge will be noticed on all
parts of the paper exposed to the vapor of iodine.

Otherwise a different and easily distinguished tinge shows itself on
the surface that has been moistened and then dried in the open air.

Machine-made papers with starchy and resinous sizing give such decided
reactions that sometimes it is possible to distinguish by the color
the portion of the paper treated with alcohol from that moistened with
water. The spot produced by alcohol takes a kind of yellow tinge; that
formed by water becomes a violet blue, more or less deep, after having
dried at an ordinary temperature. As to the spots produced by other
aqueous liquids, they approach in appearance, though not in intensity,
those occasioned by pure water. Feeble acids, or those diluted by
water, act like water; but the concentrated mineral acids, in altering
more or less the substance of the sizing, produce spots that present
differences.