um of tensions; and since by a succession of such
steps we may arrive at the order of magnitude throughout, we conclude
that this is the disposition of minimum tensions and energy.

So far the results of Laplace's hypothesis are in marked accordance with
experiment; but if we follow it out further, discordances begin to
manifest themselves. According to (52)
____ ____ ____
\/ T31 = \/ T12 + \/ T23, (53)

a relation not verified by experiment. What is more, (52) shows that
according to the hypothesis T12 is necessarily positive; so that, if
the preceding argument be correct, no such thing as mixture of two
liquids could ever take place.

There are two apparent exceptions to Marangoni's rule which call for a
word of explanation. According to the rule, water, which has the lower
surface-tension, should spread upon the surface of mercury; whereas the
universal experience of the laboratory is that drops of water standing
upon mercury retain their compact form without the least tendency to
spread. To Quincke belongs the credit of dissipating the apparent
exception. He found that mercury specially prepared behaves quite
differently from ordinary mercury, and that a drop of water deposited
thereon spreads over the entire surface. The ordinary behaviour is
evidently the result of a film of grease, which adheres with great
obstinacy.

The process described by Quincke is somewhat elaborate; but there is
little difficulty in repeating the experiment if the mistake be avoided
of using a free surface already contaminated, as almost inevitably
happens when the mercury is poured from an ordinary bottle. The mercury
should be drawn from underneath, for which purpose an arrangement
similar to a chemical wash bottle is suitable, and it may be poured into
watch-glasses, previously dipped into strong sulphuric acid, rinsed in
distilled water, and dried over a Bunsen flame. When the glasses are
cool, they may be charged with mercury, of which the first part is
rejected. Operating in this way there is no difficulty in obtaining
surfaces upon which a drop of water spreads, although from causes that
cannot always be traced, a certain proportion of failures is met with.
As might be expected, the grease which produces these effects is largely
volatile. In many cases a very moderate preliminary warming of the
watch-glasses makes all the difference in the behaviour of the drop.

The behaviour of a drop of carbon bisulphi