It therefore immediately occurs to one, Why not
return to Lenard's tube, provide a Crookes tube with
an aluminium window, and thus save the great absorp-
tion of the glass walls of the tube ? There are certain
practical difficulties in the way. The aluminium must
be very thin. Lenard used a window which we have
seen was about one nine thousandth of an inch thick,
and it was necessarily very small, in order to stand the
atmospheric pressure. An aluminium window one eighth



290 WHAT IS ELECTRICITY t

of an inch thick, or as thick as a ten-cent piece, would
absorb nearly as much as the glass walls of the present
forms of Crookes tubes, which are not more than one
sixtieth of an inch thick. Glass vessels seem at pres-
ent to be more practical than any composite form, in
which aluminium is glued to a glass-supporting vessel :
first, because they can be blown very thin, and in a shape
strong enough to withstand the atmospheric pressure ;
secondly, because the occluded air can be more effect-
ively driven off the inner walls of the vessels by heat-
ing it while it is being exhausted than it can be expelled
from a vessel of any other material.
