tric pair is like a feeble chemico-electric one of two
metals in two liquids, but without corrosion of either metal. If the
metal and liquid are each, when alone, thermo-electro-positive, and if,
when in contact, the metal increases in positive condition faster than
the liquid by being heated, the latter appears thermo-electro-negative,
but if less rapidly than the liquid, the metal appears
thermo-electro-negative.

As also the proportion of cases is small in which metals that are
positive in the ordinary thermo-electric series of metals only become
negative in the metal and liquid ones (viz., only 73 out of 286 in
weak solutions, and 48 out of the same number in strong ones), we may
conclude that the metals, more frequently than the liquids, have the
greatest thermo-electric influence, and also that the relative
largeness of the number of instances of thermo-electro-positive metals
in the series of metals and liquids, as in the series of metals only,
is partly a consequence of the circumstance that rise of temperature
usually makes substances--metals in particular--electro-positive.
These statements are also consistent with the view that the elementary
substances lose a portion of their molecular activity when they unite
to form acids or salts, and that electrolytes therefore have usually a
less degree of molecular motion than the metals of which they are
partly composed.

The current from a thermo-couple of metal and liquid, therefore, may
be viewed as the united result of difference of molecular motion,
first, of the two junctions, and second, of the two heated (or cooled)
substances; and in all cases, both of thermo- and chemico-electric
action, the immediate true cause of the current is the original
molecular vibrations of the substances, while contact is only a static
permitting condition. Also that while in the case of thermo-electric
action the sustaining cause is molecular motion, supplied by an
external source of heat, in the case of chemico-electric action it is
the motion lost by the metal and liquid when chemically uniting
together. The direction of the current in thermo-electric cases
appears to depend upon which of the two substances composing a
junction increases in molecular activity the fastest by rise of
temperature, or decreases the most rapidly by cooling.

* * * * *

AIR REFRIGERATING MACHINE.

[Illustration: IMPROVED AIR REFRIGERATING MACHINE.]

Messr