INTRODUCTION
xxi
which are different for different temperatures-2-45 gm. at 0° C.,
4.70 gm. at 10° C., and 8.70 gm. at 20° C., for these amounts
would give the air a relative humidity of 50 per cent. Now let
us assume that the temperature is 0° out of doors, and 20° in the
(heated) room. Then in the room there would be (since the air
comes ultimately from the outside) only 2·45 gm. of water in each
cubic metre of air. In order to get the correct proportion there
should be 8.70 gm. of water. Hence the air is relatively very dry
and needs 64 gm. more of water per cubic metre. Since the room
is about 7 metres long, 7 metres broad, and 4 metres high, it
contains 7 x 7 x 4 cubic metres, and the additional amount of
water required in the room is 7 x 7 x 4 × 61 gm., or nearly 14
litres. Thus if the room were hermetically closed, 1 litres of
water would have to be sprinkled about in order to secure the
proper degree of humidity. Now the room is not hermetically
closed. Let us assume that all the air in it is completely changed
in n hours; then every n hours 1 litres of water would have to
be sprinkled about or evaporated into it. I think we may assume
that through window-apertures, opening of doors, etc., the air is
completely changed every two or three hours; hence from to
of a litre of water, or a big glassful, would have to be evapor-
ated per hour. All this would roughly hold good whenever rooms
are artificially heated, and the external temperature is below 10°
C. If you were to set up a hygrometer and compare the humidity
when water is sprinkled and when it is not, you could from this
find within what time the air in the room is completely changed.
. . . This has become quite a long lecture, and the postage of the
letter will ruin me; but what wouldn't a man do to keep his dear
parents and brothers and sister from complete desiccation?
As soon as the research on evaporation was finished Hertz
turned his attention to another subject, in which he had always
felt great interest-that of the electric discharge in gases. He
had only been engaged a month upon this when he succeeded
in discovering a phenomenon accompanying the spark-discharge
which had hitherto remained unnoticed (see XII. in this
volume). But he was too keen to allow this to detain him
long he at once made plans for constructing a large secondary
battery, which seemed to him to be the most suitable means
for obtaining information of more importance. His letters
tell us how he attacked the subject.
29th June 1882.
I am busy from morn to night with optical phenomena in
rarefied gases, in the so-called Geissler tubes-only the tubes I