794 COLOUR-TESTS FOR STRYCHNIA, ETC. 
mental attempt to distinguish some of the principal alkaloids 
and active principles, whether derived from the vegetable or 
animal kingdom, from each other. 
As the first object which I had in view was to distinguish 
the poisonous alkaloids and analogous active principles from 
each other, these substances are, of course, admitted into the 
tables; but I have added to these the active principles of 
many of our aperient medicines and common articles of diet; 
and have excluded only such of those substances as were so 
strongly characterised by colour or odour as not to be pro¬ 
perly grouped with the colourless or faintly-coloured and 
inodorous alkaloids and active principles.* The table, di¬ 
vided for convenience into two, consists, as it is, of as many 
as thirty-five different substances, closely resembling each 
other in physical properties and chemical composition, and 
offering collectively a very difficult subject for tabular ana¬ 
lysis. The tables, as you have them before you, are the 
result of a long series of experiments, and were only made 
to assume their present form after a great many experimental 
groupings and transpositions. 
The first object which I had in view was to find some 
simple test which would divide the whole body of active 
principles comprised in the two tables into two principal 
groups. Concentrated sulphuric acid, as a constituent of 
the strychnia colour-tests, was obviously well adapted to 
this purpose. It has the effect of dissolving the alkaloids 
and active principles 2 vithout change of colour in the case of 
one considerable group (Table I), and with change of colour in 
the case of another considerable group (Table II). The 
first group (Table I) comprises the majority of the active 
poisonous principles—strychnia, brucia, morphia; atropine, 
picrotoxia, aconitina; and the animal principle cantharadine. 
With these and several other alkaloids and active principles, 
some of which are very harmless ones, concentrated sulphuric 
acid either produces no change of colour, or a faint yellow- 
straw or buff tint.f I speak o i cold, concentrated sulphuric acid, 
applied as in the preliminary to the colour-tests for strychnia. 
* To this statement it will be seen that there are one or two exceptions, 
such as naphthaline, which, though colourless, has the odour of tar. 
f I ought to state in this place that some of the alkaloids are very sen¬ 
sitive to heat, so that a very slight rise of temperature is followed by a 
decided development of colour. Hence it is possible that experiments made 
in a cold room in winter may yield results differing somewhat from the same 
experiments made in a warm room in the height of summer. It must also 
be borne in mind that sulphuric acid, if not free from nitric acid, may 
impart to some of the alkaloids a tint of rose, yellow or buff, sufficiently 
deep to justify the removal from Table I to Table II. It may be well, 
