coil between zinc or cadmium
terminals. Sunlight is not rich in ultra-violet light, and does not
produce anything like so great an effect as the arc light. Elster and
Geitel, who have investigated with great success the effects of light on
electrified bodies, have shown that the more electro-positive metals
lose negative charges when exposed to ordinary light, and do not need
the presence of the ultra-violet rays. Thus they found that amalgams of
sodium or potassium enclosed in a glass vessel lose a negative charge
when exposed to daylight, though the glass stops the small amount of
ultra-violet light left in sunlight after its passage through the
atmosphere. If sodium or potassium be employed, or, what is more
convenient, the mercury-like liquid obtained by mixing sodium and
potassium in the proportion of their combining weights, they found that
negative electricity was discharged by an ordinary petroleum lamp. If
the still more electro-positive metal rubidium is used, the discharge
can be produced by the light from a glass rod just heated to redness;
but there is no discharge till the glass is luminous. Elster and Geitel
arrange the metals in the following order for the facility with which
negative electrification is discharged by light: rubidium, potassium,
alloy of sodium and potassium, sodium, lithium, magnesium, thallium,
zinc. With copper, platinum, lead, iron, cadmium, carbon and mercury the
effects with ordinary light are too small to be appreciable. The order
is the same as that in Volta's electro-chemical series. With
ultra-violet light the different metals show much smaller differences in
their power of discharging negative electricity than they do with
ordinary light. Elster and Geitel found that the ratio of the
photo-electric effects of two metals exposed to approximately
monochromatic light depended upon the wave-length of the light,
different metals showing a maximum sensitiveness in different parts of
the spectrum. This is shown by the following table for the alkaline
metals. The numbers in the table are the rates of emission of negative
electricity under similar circumstances. The rate of emission under the
light from a petroleum lamp was taken as unity:--

Blue. Yellow. Orange. Red.
Rb .16 .64 .33 .039
Na .37 .36 .14 .009
K .57 .07 .04 .002

The table shows that the absorption of light by the metal has great
infl