8 INTRODUCTION. 



and this is of so great an importance, because the ne- 

 bulae which are considered to be the first stages in 

 the evolution of the celestial bodies consist, if not ex- 

 clusively, at least chiefly of Helium. 



This leads to the conception that during the deve- 

 lopment of celestial bodies from nebulae, the elements 

 were formed of Helium or of a similar substance, and 

 that these elements, later on, united for the greater 

 part with other elements, thus forming more complica- 

 ted chemical bodies. 



Bij this process also, water and rocks were formed 

 which, under the influence of the changes in the form 

 of the earth's crust, caused by cooling off, changed 

 places, so that where mountains once were, oceans now 

 spread their waters and reversely. 



Of course, the time required to bring about all these 

 changes has* been a long one ; its length can be deter- 

 mined approximately by studying the behaviour of 

 radioactive minerals. 



Radio-activity namely, proceeds with a velocity, 

 independent of external circumstances such as heat 

 etc., and this allows us to calculate, if the quantity 

 of Helium present in a mineral is known, by determi- 

 nation of the quantity of Helium yearly split off by 

 this mineral, the minimum age of it. It was thus possi- 

 ble to state positively, that Titanite or Grothitefrom 

 the Archaic Period must, at the very least, be 710 mil- 

 lion years of age, and this agrees quite well with the 

 estimate of the earth's age at 1000 millions of years, 

 derived from data, concerning the formation of the 

 great saltbeds. 



