McCretianp anp Hackerr—Secondary Radiation from Compounds. 35 
Carbon was again tested, using a pure sample, and it was found necessary to 
appreciably reduce the value of the secondary radiation given in fig. 2. The 
general result of the determinations made for elements in this period was to show 
that the part of the curve corresponding to this period is steeper than for any 
other period. This again is in complete accord with the fact that the change in 
chemical properties from element to element in this period is more marked than 
in any other period. 
GENERAL DISCUSSION. 
The work in this paper, together with that in the previous papers referred to, 
shows very clearly that the property dealt with is entirely an atomic one, and 
that the secondary @ radiation excited by an incident radiation of constant 
intensity depends in a remarkable way on the atomic weight of the element 
concerned. In a previous paper a general theory was sketched of how this 
connection may arise, and of the possible meaning of the divisions of the 
elements corresponding to the chemical periods, but the present is a suitable 
occasion to add a few more remarks on the subject. 
Whether the emission of these secondary 6 rays involves a breaking up of the 
atom in the sense that we use the term in connection with radio-activity is a point 
that is uot decided. In the previous paper the energy of the secondary radiation 
was compared with that of the primary producing it, and from these determina- 
tions a theoretical calculation enabled it to be shown that of the energy absorbed 
by any element of volume of a substance as much as 89-5 per cent. was emitted 
again as secondary B rays in the case of lead, the percentage being, of course, even 
slightly greater for bismuth and uranium, the only substances tested of higher 
atomic weight. From this result, and the method of calculation used, it is easy 
to see that an exceedingly small emission /rom a plate of lead of very easily 
absorbed secondary radiation would lead to the result that the energy of the total 
secondary radiation from any element of volume in the plate would exceed 
the energy of the radiation absorbed by that element of volume. Such a 
conclusion would, of course, prove that the effect was of the nature of true 
radio-activity ; and, moreover, would be an example of radio-activity artificially 
controlled. However, it must be stated that we have for so far no proof that 
there is this additional quantity of easily-absorbed secondary radiation. It is, 
of course, possible that the emission of the 8 particles which constitute the 
secondary radiation does not follow from, or result in, a breaking up of the 
atom in the sense that we use the term in radio-activity. The £ particles 
that are emitted as secondary radiation must, however, actually come from the 
TRANS. ROY. DUB. SOC., N.S., VOL, IX., PART III. G 
