ere pursued is also applicable to water. It
is in some sense complementary to that of the microscope, and may, I
think, materially aid enquiries conducted with that instrument. In
microscopic examination attention is directed to a small portion of
the liquid, and the aim is to detect the individual particles. By the
present method a large portion of the liquid is illuminated, the
collective action of the particles being revealed, by the scattered
light. Care is taken to defend the eye from the access of all other
light, and, thus defended, it becomes an organ of inconceivable
delicacy. Indeed, an amount of impurity so infinitesimal as to be
scarcely expressible in numbers, and the individual particles of which
are so small as wholly to elude the microscope, may, when examined by
the method alluded to, produce not only sensible, but striking,
effects upon the eye.

We will apply the method, in the first place, to an experiment of M.
Pouchet intended to prove conclusively that animalcular life is
developed in cases where no antecedent germs could possibly exist. He
produced water from the combustion of hydrogen in air, justly arguing
that no germ could survive the heat of a hydrogen flame. But he
overlooked the fact that his aqueous vapour was condensed in the air,
and was allowed as water to trickle through the air. Indeed the
experiment is one of a number by which workers like M. Pouchet are
differentiated from workers like Pasteur. I will show you some water,
produced by allowing a hydrogen flame to play upon a polished silver
condenser, formed by the bottom of a silver basin, containing ice. The
collected liquid is pellucid in the common light; but in the condensed
electric beam it is seen to be laden with particles, so thick-strewn
and minute as to produce a continuous luminous cone. In passing
through the air the water loaded itself with this matter; and the
deportment of such water could obviously have no influence in deciding
this great question.

We are invaded with dirt not only in the air we breathe, but in the
water we drink. To prove this I take the bottle of water intended to
quench your lecturer's thirst; which, in the track of the beam, simply
reveals itself as dirty water. And this water is no worse than the
other London waters. Thanks to the kindness of Professor Frankland, I
have been furnished with specimens of the water of eight London
companies. They are all laden with impurities mechan