eg. C., in 50 grammes of distilled
water. After three hours, the contents of No. 1 (containing the ointment
made with _lard_) gave _no_ iodine reaction; the contents of the other
two, however, gave traces. After eight hours no further change had taken
place. The temperature was now raised to 30-35 deg. C., and kept so for
eight hours. All three beakers now gave a strong iodine reaction, 0.2
c.c. of normal silver solution being required for each 15 grammes of the
contents of the beakers.

In addition to the iodide, some of the fatty base had osmosed through
the membrane in each case.

The next experiment was made by substituting a piece of the skin (freed
from chlorine by washing) of a freshly killed sheep for the bladder. The
ointment in No. 3 in this case was made with 10 per cent. of lard. No
reaction was obtained, at the ordinary temperature, after twelve hours,
nor after eight more hours, at a temperature of 25-30 deg. C. After letting
them stand for eight hours longer at 30-37 deg. C., a faint reaction was
obtained in the case of the ointment made with unguentum paraffini; a
still fainter with No. 3; but no reaction at all with No. 1 (that made
with lard). None of the fats passed through by osmosis. After eight
hours more, the iodine reaction was quite decisive in all cases, but no
fat had passed through even now. On titrating 20 grammes of the contents
of each beaker,

No. 1 required 0.5 c.c. of silver solution.
No. 3 " 0.5 c.c. "
No. 2 " 0.7 c.c. "

showing that the most iodine had osmosed in the case of the ointment
made with unguentum paraffini (equivalent to vaseline).

* * * * *

THE TAILS OF COMETS.

I.--If we throw a stone into the water, a wave will be produced that
will extend in a circle. The size of this wave and the velocity with
which it extends depend upon the size of the stone, that is to say, upon
the intensity of the mechanical action that created it. The extent and
depth of the water are likewise factors.

If we cause a cord to vibrate in the water, we shall obtain a succession
of waves, the velocity and size of which will be derived from the cord's
size and the intensity of its action. These waves, which are visible
upon the surface, constitute what I shall call _mechanical waves_. But
there will be created at the same time other waves, whose velocity of
propagation will be much greater than th