Reactions were also obtained with old human blood. Thus stains on linen from several days to two months in age, when treated with dilute solutions of common salt gave a solution which yielded a precipitate with the prepared rabbit’s serum. No reaction was obtained, however, with the preparation from a blood-stain nine years old or with that from blood which had been dried in a high temperature.
It was also found that the specific sera could be evaporated in a vacuum without losing their activity, and that the dried residues could be preserved in sealed tubes in the dark, and mixed with water when required for use.
Other investigators showed that it was possible to separate the active agent by adding magnesium sulphate to the serum, and that the precipitate could be dried and kept for a long period. By dissolving it in water at any time a liquid with the specific properties of the original serum could then be obtained.
Later work has shown that this serum test is not quite so absolute as was at first believed. Thus, if the blood serum to be tested be used in too concentrated a form it may give a reaction with a serum that is not specific to it, though even in that case the precipitate will only appear slowly and its amount will be insignificant in comparison with that obtained when the two liquids correspond.
The error is obviated by using extremely dilute solutions for the test, and when proper precautions are taken a solution of normal blood serum containing one part in 1,000 invariably gives a reliable reaction with its corresponding prepared serum. In more concentrated solutions there is an abundant deposit at the bottom of the tube within thirty minutes, whereas in the case of sera, which are not specific to the prepared serum, the formation of precipitate does not begin until the tube has stood for an hour or more.
 |  |  | ||
| FIBRES OF CHINESE SILKS, SHOWING CROSS SECTION | KANGAROO’S HAIR | HUMAN HAIR | ||
 | ||||
| A. Hair of a Cat B. Hair of a Dog By kind permission of Messrs. Scott Greenwood & Co. | ||||
An interesting exception to the rule is that the serum from the blood of anthropoid apes gives a pronounced reaction with serum that has been made specific for human blood, and vice versa.
As it is not possible to carry out control tests with an indefinite number of animals a positive result obtained in the examination of a particular stain justifies a report that the blood was (e.g.) probably human blood and certainly not that of any common domestic animal.
On the other hand, the results of a negative test justify a much more positive statement.
Thus on the first occasion in which evidence was given as to the results of this test, which was in a criminal case in France in 1902, the prisoner had asserted that certain incriminating stains had been caused by the blood of a rabbit.