BROWN— THE AGE OF THE EARTH. 285 



age of the earth is practically nil and one must turn to methods outside 

 the range of the astronomer's work. 



A further difficulty may be mentioned. Evidence is accumulating 

 that there is widely extended diffuse matter in space, some of which 

 is visible and some of which is only evident on account of the 

 obscuration of light which it causes. It therefore seems highly prob- 

 able that the solar system in the course of several hundred million 

 years may have passed through one or several such clouds. These 

 would have effects, which from theory are well known, such as 

 diminishing the mean distances of the planets from the sun, the circu- 

 larization of their orbits, possible changes in the total angular mo- 

 mentum of the system, and other effects such as the possible forma- 

 tion of comets and the production of glacial and interglacial periods. 

 At present, however, the consequences of this hypothesis are still in 

 the range of speculation and need to be worked out in considerable 

 detail before any arguments can be built on it. It may, however, be 

 stated that such a hypothesis would have the general tendency of 

 increasing the age of the earth as estimated from other sources. 



