COLOUR-TESTS FOR STRYCHNIA, ETC. 241 
what circumstances and in what degree it will be necessary 
to inquire. 
The occasional failure of the colour-tests for strychnia 
from slight and obscure causes is not to be denied. It is 
proved by the experiments already detailed, and is admitted 
by all experienced chemists. But it should be understood 
that this objection has only a limited application. It goes 
only to the extent of impairing the value of the evidence 
derived from a negative result, if the colour-tests fail when 
applied to a spot acted on by sulphuric acid, their failure 
is not conclusive evidence of the absence of the poison. It 
affords a strong probability of such absence, but nothing 
more. 
The reactions of the colour-tests for strychnia are certainly 
transient. They are not like the permanent white, crys¬ 
talline, or coloured precipitates afforded by most chemical 
tests. But the objection on this ground is also of limited 
application. In experiments with the colour-tests made in 
the manner described in a former communication, we obtain 
two classes of results. We may get a mere trace or “ flash” 
of blue, or we may obtain a deep, rich, blue colour, lasting for 
several seconds, for half a minute, or even for forty-five 
seconds, and succeeded by the other tints already described. 
Now those who maintain the value of the colour-tests for 
strychnia do not pretend that a mere flash of colour affords 
such evidence of the presence of strychnia as could be used 
with safety in a court of law; but they would probably be 
found to assert with equal confidence that a deep, rich, blue 
colour, developed by the colour-producing tests for strychnia, 
and continuing for several seconds in a spot previously 
treated with sulphuric acid without change of colour, and 
then succeeded by other colours, affords good evidence of the 
presence of strychnia. Though the blue colour soon passes 
away, there is ample time afforded for its complete re¬ 
cognition. 
In answer to the objection that the sucession of colours is 
difficult to describe or define, it will suffice to state that 
though it may not be possible by any words that we can use 
to describe the successive colours developed by the colour- 
tests, it is not necessary to do so. For the first deep, rich, 
blue colour, characterised by the property of want of perma¬ 
nence, is in itself so remarkable as to constitute a most 
valuable and trustworthy test for strychnia. 
The colour-tests for strychnia are certainly open to the 
objection that the colour-developing substances are them¬ 
selves coloured. The peroxide of lead is puce coloured, the 
xxxv. 16 
