210 
The Keys of Death . 
[March, 
that animals placed in an atmosphere of pure oxygen are 
thrown into a morbid state, which, if the experiment is 
sufficiently prolonged, terminates fatally. Here, therefore, 
we see that dilution converts oxygen from a poison to a 
necessary of life. Or take sulphuric acid. If swallowed in 
a concentrated state it destroys all parts of the system 
which it touches. Dilute it largely with water, and it be- 
comes a pleasant summer beverage of tonic properties. 
Open an animal which has swallowed such dilute acid, and 
examine the oesophagus and stomach with the most power- 
ful microscope, and you will find no trace of those pheno- 
mena caused by the undilute acid. The results differ not 
in extent, not in degree, but in kind. These instances, 
which might be indefinitely multiplied, prove that the idea 
of poison is not essentially qualitative. 
Let us now examine the second notion which has been 
brought forward to help us to a precise definition of poisons. 
It is alleged that whilst articles of food undergo decompo- 
sition in the stomach, poisons are not thus decomposed, but 
are absorbed unchanged, exist in the tissues unchanged, 
and in cases of recovery are finally expelled still in the 
same state in which they were first ingested. This expla- 
nation sins in the first place by implying that all articles 
taken into the system must be either food or poison. But, 
further, will chromic acid undergo no change if taken inter- 
nally ? How will the chlorides of arsenic or of cyanogen 
behave ? There is again a liquid, neither rare nor entirely 
strange to chemists, which agrees with the features here 
ascribed to poisons as well as does alcohol. This liquid, if 
swallowed either in large or small doses, remains undecom- 
posed in the stomach, is taken up by the absorbents, and 
enters the blood and the tissues, still without having under- 
gone decomposition, and is finally eliminated as such in the 
urine and the perspiration. If, therefore, the above defini- 
tion of poison be corredt, this liquid is a poison ; but if it is 
found by common experience harmless and necessary, what 
of the definition ? The liquid in question is water, which 
has never been proved to suffer decomposition in the system, 
and which therefore ought to be a poison ! 
Leaving these discussions for more profitable considera- 
tions, we find included under the general name of “ poisons ” 
two classes of bodies having little in common save their 
injurious effedts upon the living organism into which they 
are introduced. We have, on the one hand, certain fer- 
ments — “ germs,” or by whatever other name they may be 
