CHEMICAL SCIEXC2. 253 



visible. The followirg experiments, made in a judicial investigation, 

 furnish us with the following facts : 



1st. The surface of paper sized in the ordinary way, or letter paper, no 

 longer presents with certain reactions the same uniformity where it has been 

 either accidently moistened in several places by various liquids, or left in 

 contact for a certain time with agents capable of removing or destroying 

 the characters which have been traced on it with ink. 



2d. The application of a thin layer of gum, of starch, or farina, of gela- 

 tine, or fish-glue, with a view of sizing certain parts of the paper, or of 

 causing certain bodies to adhere to it momentarily, is detected by an action 

 similar to that which shows paper to have lately been blotted by the con- 

 tact of liquids. 



3d. The heterogeneousness of the pulp of the papers, and the kind of size 

 with which they are impregnated, lead to differences in the results which 

 are observed with the same chemical reagents. We shall now examine 

 each of these propositions, and describe the means which we have employed 

 in endeavoring to solve questions of so high a degree of interest. 



1st. The homogeneousness of sized paper not partially altered by the 

 contact of liquids (water, alcohol, salt-water, vinegar, saliva, tears, urine, 

 acid- salts, and alkaline salts) is demonstrated by the uniform coloration 

 which this surface takes on being exposed, if not wholly, at least in various 

 parts, to the action of the vapor of iodine disengaged at the ordinary tem- 

 perature from a flask containing a portion of that metalloid. "When the 

 surface of paper not stained by any of the above-mentioned liquids is 

 exposed to the action of this vapor for three or four minutes in a room 

 the temperature of which is about 60 F., a uniform yellowish, or light- 

 brownish yellow, coloration is noticed on the whole extent exposed to the 

 vapor of iodine ; in the contrary case, the surface which has been moist- 

 ened, and afterwards dried in the open air, is perfectly distinguished by a 

 different and well-circumscribed tint. On the papers into whose paste 

 starch and rosin have been introduced, the stains present such delicate 

 reactions that we may sometimes distinguish by their color the portion of 

 paper which has been moistened with alcohol from that what has been 

 moistened with water. The stain produced by alcohol takes a bistre-yel- 

 low tint ; that formed by water is colored of a more or less deep violet 

 blue, the desiccation having been effected at the ordinary temperature. 

 For the stains occasioned on these same papers by other aqueous liquids, 

 the tint, apart from its intensity, resembles that of the stains of pure water. 

 The feeble or dilute acids act like water on the surface of the same paper 

 containing starch in its paste ; but the concentrated mineral acids, by 

 altering more or less the substances which enter into the composition of 

 the latter, give test to the stains which present differences. We are always 

 able to recognize by the action of the vapor of iodine the parts of the paper 

 which have been put in contact with chemical agents, the energy of which 

 has been arrested by washing in cold water. We were able, on several 



