nothing more than a variety of the last. If a poison, in combining with
another substance, acquire greater solubility, it also generally
acquires greater activity, and _vice versa_: Morphia, itself almost
inert, because insoluble, becomes active by uniting with acids, for they
render it very soluble: Baryta as a very active poison, becomes quite
inert by uniting with sulphuric acid, for the sulphate of baryta is
altogether insoluble.
In regard to the influence of chemical combination two general laws may
be laid down. One is, that _poisons which only act locally, have their
action much impaired or even neutralized, in their chemical
combinations_. Sulphuric acid and muriatic acid on the one hand, and the
two fixed alkalis on the other, possess a violent local action; but if
they are united so as to form sulphates or muriates, although still very
soluble, they become merely gentle laxatives. But the case is altered if
either of the combining poisons also act by entering the blood. For the
second general law is, that _the action of poisons which operate by
entering the blood, although it may be somewhat lessened, cannot be
destroyed or altered in their chemical combinations_. Morphia acts like
opium if dissolved in alcohol or fixed oil; if an acid be substituted as
the solvent, a salt is formed which is endowed with the same properties:
The sulphate, muriate, nitrate, acetate of morphia all act like opium.
Strychnia, arsenic, hydrocyanic acid, oxalic acid, and many more come
under the same denomination: Each produces its peculiar effects, with
whatever substance it is combined, provided it do not become insoluble.
Mr. Blake has recently laid down what may be considered a branch or
corollary of the second of these general propositions, and has confirmed
it by many appropriate experimental facts,—namely, that _the salts of
the same base produce the same actions, independently of the acids with
which they are combined_.[50] The law, however, is a more general one,
as given above, and was stated in former editions of the present work.
It applies not only to bases, but likewise to acids, such as the
hydrocyanic, oxalic, arsenious, and arsenic acids, and also to neutral
organic principles which act through the blood, such as picrotoxin,
colocynthin, elaterin, and narcotin.
The same author considers it to be also a probable conclusion from a
variety of experiments on the salts of various bases, that _those salts
which are isomorphous, or possess the same crystalline form, are closely
allied in action_.[51]