SUMMARY, SALTS OF AMMONIA. 173 
Cuapr. VII. 
why we should reject it as incredible. Prof. Donders, 
of Utrecht, informs me that from experiments formerly 
made by him and Dr. De Ruyter, he inferred that less 
than the one-millionth of a grain of sulphate of atro- 
pine, in an extremely diluted state, if applied directly 
to the iris of a dog, paralyses the muscles of this organ. 
But, in fact, every time that we perceive an odour, we 
have evidence that infinitely smaller particles act on 
our nerves. When a dog stands a quarter of a mile to 
leeward of a deer or other animal, and perceives its 
presence, the odorous particles produce some change in 
the olfactory nerves; yet these particles must be in- 
finitely smaller * than those of the phosphate of am- 
monia weighing the one-twenty-millionth of a grain. 
These nerves then transmit some influence to the brain 
of the dog, which leads to action on its part. With Dro- 
sera, the really marvellous fact is, that a plant without 
any specialised nervous system should be affected by 
such minute particles; but we have no grounds for 
assuming that other tissues could not be rendered as 
exquisitely susceptible to impressions from without if 
this were beneficial to the organism, as is the nervous 
system of the higher animals. 
* My son, George Darwin, has 
calculated for me the diameter of 
a sphere of phosphate of ammonia 
(specific gravity 1°678), weigh- 
ing the one-twenty-millionth of 
a grain, and finds it to be yy of 
an inch. Now, Dr. Klein informs 
me that the smallest Micrococci, 
which are distinctly discernible 
under a power of 800 diameters, 
are estimated to be from ‘0002 to 
‘0005 of a millimetre — that is, 
from soho0 tO re/ooo Of an inch 
—in diameter. Therefore, an ob- 
ject between 3; and = of the 
size of a sphere of the phos- 
phate of ammonia of the above 
weight can be seen under a high 
power; and no one supposes 
that odorous particles, such as 
those emitted from the deer in 
the above illustration, could be 
seen under any power of the mi 
croscope. 
