112 TRANSACTIONS OF THE 
elements. During this same period of disintegration, 
eight atoms of helium have appeared, so that the gross 
difference between an atom of uranium and an atom of 
lead is 8 atoms of helium. The most important point to 
be noticed here is that by this decomposition of uranium 
to form lead, with the simultaneous production of 8 atoms 
of helium, the atomic weight of the lead is put at 206.2, 
i. e., uranium of atomic weight 238.2 loses 8 atoms of he- 
lium, of atomic weight 4, leaving a material of atomic 
weight 206.2. Now the atomic weight of lead has long 
been known to be 207.2, and hence one of two conclusions 
is inevitable. Either the material resembling lead, and 
which is always found in radioactive minerals, is not lead, 
or, it is possible to have a substance of different atomic 
weight from lead, but yet showing all the chemical and 
physical properties of lead. In other words, the atoms 
of lead are not all exactly alike. Lead of undoubted radio- 
active origin has been very carefully examined, both in 
the metallic form and in the form of salts. The spectro- 
graphs of radio lead and of ordinary lead are exactly 
alike, and the molar solubilities of the salts are also 
identical. When, however, these solubilities are expressed 
in grams of salt per liter, the solubility of the radio lead 
is less, and by an amount agreeing with its lower atomie 
weight. Actual atomic weight determinations on the 
purest radio lead to be obtained give an atomic weight of 
206.08, markedly less than that called for by the calcula- 
tion made above. If, however, the radio lead has origin- 
ated from radium, of atomic weight 225.96, by loss of 5 
helium atoms, the atomic weight of lead should be 205.96, 
a still lower value. Assuming that radio lead has arisen 
equally from uranium and from radium, the average 
atomic weight becomes 206.07, a result in surprising 
agreement with the experimental data. The interesting 
question arises here, why has radium an atomic weight 
of 225.96? If radium is a decomposition product of uran- 
ium, by loss of 3 helium atoms, its atomic weight should 
be 226.2. Has it then, during this disintegration, given off 
enough beta particles to change its atomic weight by 
0.1%? This is hardly probable, since the number of elec- 
trons in a given atom seems to be of the order of its atomic 
weight. Is there, then another radium, not originating 
