So minute are the parts of the elementary bodies in their ultimate
state of division, in which condition they are usually termed _atoms_,
as to elude all our powers of inspection, even when aided by the most
powerful microscopes. Who can see the particles of gold in a solution
of that metal in _aqua regia_, or those of common salt when dissolved
in water? Dr. Thomas Thomson has estimated the bulk of an ultimate
particle or atom of lead as less than 1/888492000000000th of a cubic
inch, and concludes that its weight cannot exceed the 1/310000000000th
of a grain.
This curious calculation was made by Dr. Thomson, in order to show to
what degree Matter could be divided, and still be sensible to the eye.
He dissolved a grain of nitrate of lead in 500,000 grains of water,
and passed through the solution a current of sulphuretted hydrogen;
when the whole liquid became sensibly discoloured. Now, a grain of
water may be regarded as being almost equal to a drop of that liquid,
and a drop may be easily spread out so as to cover a square inch of
surface. But under an ordinary microscope the millionth of a square
inch may be distinguished by the eye. The water, therefore, could be
divided into 500,000,000,000 parts. But the lead in a grain of nitrate
of lead weighs 0·62 of a grain; an atom of lead, accordingly, cannot
weigh more than 1/810000000000th of a grain; while the atom of sulphur,
which in combination with the lead rendered it visible, could not
weigh more than 1/2015000000000, that is, the two-billionth part of a
grain.--_Professor Low_; _Jameson’s Journal_, No. 106.