= Solution
\b[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>.<Identification of blood by the National Institute of Justice>[Unit II, §§3–6: historical development, microscopic examination, crystal tests and spectral methods]{ref}
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.<Confirmatory blood tests by the National Institute of Justice>[“Confirmatory Tests”: crystal formation versus human-origin testing with specific antisera]{ref}
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