ELECTRIC RADIATIONS—BRAGG. 213 
a neutral pair. Now, J. J. Thomson has just shown (Phil. Mag., 
May, 1907) that the canal rays consist of positive electrons, which 
may be H or H, or He, according to circumstances, and that these 
appear no matter what the material is in the tube. It will be remem- 
bered that Villard (Ions, Electrons, Corpuscules, p. 1022) was so im- 
pressed with the continual presence of hydrogen in vacuum tubes that 
he supposed the cathode particles to consist of hydrogen, until ac- 
curate measurements of the mass and velocity of the particles were 
made. He was largely influenced by the reducing action of the rays. 
After all, it may be that H is produced where they strike, and that 
Villard’s observations can be explained in this way. Sir William 
Ramsay (Journ. Chem. Soc., May, 1907) has shown that there is an 
excess of hydrogen in water decomposed by radium emanation; but 
the circumstances are too complicated to make the connection more 
than a possibility at present. 
H. W. Schmidt has arrived at the conclusion (Phys. Zeit., June, 
1907) that the “secondary” radiation caused by @ rays striking 
aluminium consists of scattered primary rays. This is in agreement 
with the argument stated above. He has also shown that undeflected 
B particles lose no speed in passing through a metal plate. This im- 
ples either that the energy required to produce ions does not come 
from the £ particle or that the 8 particle does not produce ions until 
it is deflected. ‘There seem several difficulties in the way of the latter 
supposition, though it is, of course, a possibility. It seems to me 
probable that the @ particle rarely produces more than one ion from 
a traversed molecule, but that an a particle may produce many, and 
that initial recombination is to be explained in this way. Kleeman 
has pointed out, in his Royal Society paper, that an a particle which 
has lost several ions has not yet been observed; but it is to be remem- 
bered that such a molecule would probably dissociate at once, and 
it is well known that the a particle does produce dissociation. 
Note.—In a supplementary paper contributed to the Royal Society of South 
Australia and dated January 2, 1908, Bragg has described certain experiments 
which he has performed with the object of testing the hypotheses of the original 
paper. 
He argues that on the ether-pulse theory there should be perfect symmetry 
in the secondary radiations on the two sides of a thin plate through which a 
stream of y rays is passed normally. He finds, however, that such an expecta- 
tion is completely contradicted by experiment. 
On the other hand, he shows that on the neutral pair theory the quantity of 
secondary cathode radiation which is excited on the near side of the plate, i. e., 
the side on which the rays are incident, should depend on the atomic weight 
of the material of the plate according to the same law as that which holds for 
the secondary cathode radiation due to f rays. 
It is well known that this is actually the case. Again, he shows that on the 
Same neutral pair theory the quantity of secondary radiation which is excited 
on the far side of the plate, i. e., the side from which the rays emerge, should 
