"2d. The applications made to the surface of a sheet of paper, with a view of covering it again at certain parts with a fine layer of gum, gelatine, starch or flour paste, or in other places to cause other sheets of paper to adhere, may be recognized not only by the reflection of light falling upon the paper inclined at a certain degree of obliquity, and by the transmission of light through the paper, but also by the varying action which the vapor of iodine exerts on the surface which is not homogeneous. Papers containing starch and resin are more powerfully acted upon by this vapor than papers of a less complex composition. Both in the parts covered with starch, or paste flour, are colored in a few minutes of a violet blue; but with starched papers alone a more intense coloration is manifest on the places covered again with a thin layer of gum arabic, size or gelatine. By looking, then, on the surface of the paper, held somewhat obliquely to incidental light, we distinguish clearly, by their different aspects, the parts on which these various substances have been applied. The vapor of iodine, in condensing at the ordinary temperature on the surface of the papers to which any kind of size has been applied in various places, produces differences which are most commonly well recognized by the greater or less transparence of the paste of the paper. 3d. The heterogeneousness of the pulp of the various papers of commerce, and the nature of the size with which they are penetrated, cause differences, either in the coloration which the surface of these papers takes when exposed to the vapor of iodine, or in the tint which is manifested in the portions of the size deposited in certain portions of that surface; thus, papers with starched pulp generally turn brown, or blue, according to the amount of water that remains in their interstices; other papers turn yellow only under the influence of the vapor of iodine, and the parts which have received superficially a layer of another agglutinative body resist this action for a certain time, and are distinguished from the parts of the paper which are not covered with it." My own investigations confirm to a great extent the value of these experiments and the accuracy of the deductions, in so far as they relate to "linen" paper; but they do not always obtain when made in connection with paper of inferior grades. It is also true that dry paper is affected differently under the influence of the vapor of iodine, as would be paper which had been moistened and then dried; but the part which had been moist assumes the color of blue-violet, while unaltered paper assumes a yellow- brown color. Even when the paper thus treated is moistened all over with water, there will be a difference, for those parts which had been before moistened, will appear a dark violet-blue, while the other parts will show a plain blue coloration.