2. The menstrual discharge may be readily mistaken for that due to violence, as the two kinds of blood cannot be distinguished.
3. The red juice of fruits and grease spots have been mistaken for marks of blood and seminal stains on linen.
The identification of blood-stains is not difficult when the stain occurs on pieces of white linen; but when, as it not infrequently happens, they have to be detected on the coarse, dirty, often stinking linen of the poor, the task becomes somewhat more difficult. The same may be said with regard to seminal spots. As a means of diagnosis in stains due to semen, the appearance and smell of the stains are of no assistance whatever. The microscope will alone give any trustworthy evidence as to the nature of the stain; and even here a caution must be added—for the fact is beyond doubt that the semen even of a healthy young man varies much, and is scarcely ever twice alike, so that the absence of spermatozoa is no proof that the spot is not seminal in its origin.
The following are the tests used for the detection of semen:
1. Characteristic smell when the spot is moistened.—This test is of no use, for the reasons before stated.
2. Appearance when held to the light.—As uncertain as the preceding.
3. Doubtful spots upon cotton or linen—not upon wool, which usually contains sulphur—should be cut out and moistened with a few drops of oxide of lead, dissolved in liquor potassæ, and then dried at a temperature of 68° F. The stain in a few minutes becomes of a dirty yellow or sulphur-yellow colour. This change in colour proves that the mark is not a seminal stain. Semen does not contain albumen. This test only shows that the stain is not caused by albuminous compounds, which contain sulphur; but it does not follow therefore that the spot must be seminal, for marks made by gum, dextrine, and some other substances of a like nature are not changed in colour.
4. The Microscope.—This is by far the most reliable test, but care is required in its manipulation.
(a) The cloth must not be rubbed between the fingers, as the spermatozoa may be damaged by the operation.
(b) The suspicious spot on the linen should be carefully cut out and placed in a clean watch-glass or small porcelain vessel, and then moistened with a small quantity of distilled water. The cloth may be gently moved about in the water with a glass rod, and gentle pressure made so as to thoroughly wet the cloth, which, in most cases, will be accomplished in about a quarter of an hour. A single drop should now, by gentle pressure with the fingers, be squeezed on to a clean slide, and then placed under the microscope.