mer Lenard rays. In air at atmospheric
pressure the Lenard rays spread out very diffusely. If the aluminium
window, instead of opening into the air, opens into another tube which
can be exhausted, it is found that the lower the pressure of the gas in
this tube the farther the rays travel and the less diffuse they are. By
filling the tube with different gases Lenard showed that the greater the
density of the gas the greater is the absorption of these rays. Thus
they travel farther in hydrogen than in any other gas at the same
pressure. Lenard showed, too, that if he adjusted the pressure so that
the density of the gas in this tube was the same--if, for example, the
pressure when the tube was filled with oxygen was 1/16 of the pressure
when it was filled with hydrogen--the absorption was constant whatever
the nature of the gas. Becker (_Ann. der Phys._ 17, p. 381) has shown
that this law is only approximately true, the absorption by hydrogen
being abnormally large, and by the inert monatomic gases, such as helium
and argon, abnormally small. The distance to which the Lenard rays
penetrate into this tube depends upon the pressure in the discharge
tube; if the exhaustion in the latter is very high, so that there is a
large potential difference between the cathode and the anode, and
therefore a high velocity for the cathode rays, the Lenard rays will
penetrate farther than when the pressure in the discharge tube is higher
and the velocity of the cathode rays smaller. Lenard showed that the
greater the penetrating power of his rays the smaller was their magnetic
deflection, and therefore the greater their velocity; thus the greater
the velocity of the cathode rays the greater is the velocity of the
Lenard rays to which they give rise. For very slow cathode rays the
absorption by different gases departs altogether from the density law,
so much so that the absorption of these rays by hydrogen is greater than
that by air (Lenard, _Ann. der Phys._ 12, p. 732). Lenard (_Wied. Ann._
56, p. 255) studied the passage of his rays through solids as well as
through gases, and arrived at the very interesting result that the
absorption of a substance depends only upon its density, and not upon
its chemical composition or physical state; in other words, the amount
of absorption of the rays when they traverse a given distance depends
only on the quantity of matter they cut through in the distance.
McClelland (_Proc. Roy. Soc._ 61, p. 227) show