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Incoming links: Identification of blood by the National Institute of Justice
Establish that the stain contains blood; then state separately whether its human origin can be established. In 1867, a positive blood test could not reliably distinguish human blood from that of another mammal.
First record the position, extent, colour and underlying material of the stain. Keep a portion untouched, take a small scraping or cutting for examination, and test an unstained portion of the same material alongside it. The comparison is important: a coloured fabric, iron oxide or a vegetable colouring substance may resemble dried blood, and the material itself can interfere with a chemical test. Brown colour alone proves nothing.
For an examination practicable in 1867, use complementary observations:
- Moisten a small portion with an appropriate aqueous medium and examine it by microscopy. Recognisable red blood cells support the identification. Oval, nucleated red blood cells would point towards certain non-mammalian sources; human red blood cells are normally non-nucleated discs. Drying and extraction distort the cells, however, and their measured diameters overlap those of other mammals. An apparent match with human red blood cells therefore does not establish human origin.
- Apply the historical haemin crystal test to a separate portion: gently heat the material with acetic acid and a little sodium chloride on a covered microscope slide, then look for the characteristic brown crystalline product. A positive result supports the presence of a hemoglobin derivative. It does not identify the species.
- Where a suitable spectroscope is available, examine an extract for the characteristic absorption spectrum of hemoglobin and the changes produced by altering its chemical state. This adds a different kind of evidence, but the absorption spectrum is again shared by the blood of different species. Spectroscopy of blood was already developing before this examination; it should not be confused with later DNA profiling.[ref]Identification of blood by the National Institute of Justice. Unit II, §§3–6: historical development, microscopic examination, crystal tests and spectral methods
These observations should agree before the examiner expresses a positive conclusion. Failure to recover intact red blood cells does not exclude an old bloodstain; conversely, a colour reaction without suitable controls is weak evidence. A defensible period report would distinguish “blood identified, consistent with mammalian origin” from “human blood proved.” The second conclusion does not follow from the first.
For comparison, later precipitin tests use antibodies against human proteins to investigate species of origin. Modern DNA profiling can investigate the donor, subject to preservation and contamination limits. These are later developments, not procedures to attribute to an Oxford candidate in 1867.[ref]
Confirmatory blood tests by the National Institute of Justice. “Confirmatory Tests”: crystal formation versus human-origin testing with specific antisera