ADDRESSES 65 
was specially fusible or soluble, and these liquid parts, under 
the kneading strains of the earth, worked outward and initiated 
volcanic action, the radioactive matter partaking in the ascen- 
sion and thus concentrating toward the surface. Life con- 
ditions arose early, and no doubt life soon followed and par- 
ticipated in the processes of growth. Thus slowly the earth 
grew to maturity and merged into the later history recorded 
in the known strata. The whole may be shortly summarized 
as a slow-growing earth-body, bearing a slow-growing hydro- 
sphere, enwrapped in a slow-growing atmosphere. 
Critical comparison of these two series of pictures must be 
left to special students of the earth and to time. Certain 
vital features of recent determination which will largely control 
future opinion relative to the genesis of the earth, are worthy 
of special attention: (1) the undeniable evidence of an alterna- 
tion of cold and warm climatic stages following one another 
from the earliest date to which climatic evidences reach, now 
raised to demonstration by a long list of competent observers; 
(2) decisive evidence of the deep differentiation of the specific 
gravity of the earth-material, brought out by Hayford’s analy- 
sis of geodetic data; (3) evidence of the concentration of radio- 
active material in the outer part of the earth, brought out by 
Strutt, Joly, Holmes, and others; (4) the determination of 
the earth’s high rigidity, initiated by Kelvin and others, sup- 
ported by seismic data gathered by a host of observers, support- 
ed also by the pendulum studies of Hecker, and now brought to 
a full demonstration by the brilliant co-operative work of 
Michelson, Gale and Moulton on new lines. All these new 
developments are welcome from the viewpoint of the planetesi- 
mal hypothesis. Whether the terms of the older hypotheses 
can be revised to fit them or not, it does not fall to me to say. 
The foregoing relates to the origin of our present planets 
and satellites. A few words may be added relative to other 
forms of planetesimal genesis, and to the origin of the com- 
ets and meteorites. Elsewhere I have endeavored to show 
that, on the surface of a great rotating body of hot gases, the 
molecular activities necessarily give rise to krenal flights of 
molecules, i, e. elliptical excursions outward from the body, 
which are checked by gravity and returned to it without the 
collisions that usually terminate molecular flights within a body 
of gas. These krenal flights are especially promoted in the 
