8 
MENDENHALL. 
eral be established by experiment, the marvelous accuracy 
with which its predictions are verified has long ago placed 
it far in advance of other sciences. An inquiry into the 
cause of this excellence will not show that the logic of the 
astronomer is any more rigorous than that of many others 
engaged in scientific research, but rather that the premises 
on which he reasons are simpler, and, what is of greater im¬ 
portance, more nearly sufficient. Until a very recent period 
in its history, astronomy, although dealing with matter, has 
been concerned almost entirely with only one of its many 
properties. The one property thus far assumed to be com¬ 
mon to all matter is that long known but still mysterious 
attraction in virtue of which there exists a stress between 
every particle and every other particle in the universe, ac¬ 
cording to a law the discovery and exposition of which justly 
entitles Newton to be considered the greatest philosopher of 
all ages. It happens that the hundreds and possibly thou¬ 
sands of other properties posses£ed by or inherent in matter 
have little if any influence on the dynamics of masses widely 
separated from each other, and therefore a knowledge of the 
law of gravitation seems to be sufficient to enable the astron¬ 
omer, having, of course, obtained the necessar}^ data from 
observation, to trace the paths of the planets and to foretell 
the configuration of the heavens many years in advance. 
Within the past twenty-five years, however, the splendid 
discovery of spectroscopy, aided by great improvements in 
photography, has given rise to a new astronomy, known as 
physical, as distinguished from gravitational astronomy. 
The new science deals with a matter of many properties, 
some of which are but little understood. While its conclu¬ 
sions are of vital importance and of intense interest, they 
result from deductions in which the premises are insufficient 
and are proportionately uncertain. The new astronomy 
must for a long time abound in contradictions and contro¬ 
versies until, and largely through its development, we shall 
possess a knowledge of the properties of matter when sub¬ 
jected to conditions differing enormously from those with 
