September 1, 1905.] 



SCIENCE. 



265 



condensation of a rarefied gas, was sup- 

 posed to be the sole cause of the sun's high 

 temperature. 



Since the mass of the sun is known, the 

 total amount of the heat generated in it, 

 in whatever mode it was formed, can be 

 estimated with a considerable amount of 

 precision. The heat received at the earth 

 from the sun can also be measured with 

 some accuracy, and hence it is a mere mat- 

 ter of calculation to determine how much 

 heat the sun sends out in a year. The 

 total heat which can have been generated 

 in the sun divided by the annual output 

 gives a quotient of about 20 millions. 

 Hence it seemed to be imperatively neces- 

 sary that the whole history of the solar 

 system should be comprised within some 20 

 millions of years. 



This argument, which is due to Helm- 

 holtz, appeared to be absolutely crushing, 

 and for the last forty years the physicists 

 have been accustomed to tell the geologists 

 that they must moderate their claims. But 

 for myself I have always believed that the 

 geologists were more nearly correct than 

 the physicists, notwithstanding the fact 

 that appearances were so strongly against 

 them. 



And now, at length, relief has come to 

 the strained relations betv/een the two 

 parties, for the recent marvelous discov- 

 eries in physics show that concentration of 

 matter is not the only source from which 

 the sun may draw its heat. 



Radium is a substance which is perhaps 

 millions of times more powerful than dyna- 

 mite. Thus it is estimated that an ounce 

 of radium would contain enough power to 

 raise 10,000 tons a mile above the earth's 

 surface. Another way of stating the same 

 estimate is this: the energy needed to tow 

 a ship of 12,000 tons a distance of six thou- 

 sand sea miles at 15 knots is contained in 

 22 ounces of radium. The Saxon probably 

 burns five or six thousand tons of coal on 



a voyage of approximately the same length. 

 Other lines of argument tend in the same 

 direction. 



Now we know that the earth contains 

 radioactive materials, and it is safe to as- 

 sume that it forms in some degree a sample 

 of the materials of the solar system ; hence 

 it is almost certain that the sun is radio- 

 active also. 



This branch of science is as yet but in its 

 infancy, but we already see how unsafe it 

 is to dogmatize on the potentialities of 

 matter. It appears, then, that the physical 

 argument is not susceptible of a greater 

 degree of certainty than that of the geol- 

 ogists, and the scale of geological time re- 

 mains in great measure unknown. 



I have now ended my discussion of the 

 solar system, and must pass on to the wider 

 fields of the stellar universe. 



Only a few thousand stars are visible 

 with the unaided eye, but photography has 

 revealed an inconceivably vast multitude 

 of stars and nebujce, and every improve- 

 ment in that art seems to disclose yet more 

 and more. It seems useless to consider 

 whether the number of stars has any limit, 

 for infinite number, space and time tran- 

 scend our powers of comprehension. We 

 must then make a virtue of necessity, and 

 confine our attention to such more limited 

 views as seem within our powers. 



A celestial photogi^aph looks at first like 

 a dark sheet of paper splashed with white- 

 wash, but further examination shows that 

 there is method in the arrang'^ment of the 

 white spots. Thus there is order of some 

 sort in the heavens, and, although no rea- 

 son can be assigned for the observed ar- 

 rangement in any particular case, yet it 

 is possible to obtain general ideas as to the 

 succession of events in stellar evolution. 



Besides the stars there are numerous 

 streaks, wisps and agglomerations of nebu- 

 losity, whose light we know to emanate 



