J 4 Proceedings of the Royal Society of Victoria. 



inao;nitude of the sun it has been found that the diameter 

 must contract about 220 feet a year to produce all the heat 

 which it radiates. This I'ate of contraction diminishes as the 

 sun grows smaller, at such a rate that in five millions of 

 years it will be reduced to one half its present volume. At 

 the present rate of i-adiation the sun will be as dense as the 

 earth in about twelve millions of years. As to the past, it 

 has been calculated that the heat evolved by conti'action 

 from an infinite size or by the falling together of all the parts 

 of the sun from an infinite distance would only have been 

 sufficient to last eighteen millions of years at the present 

 rate of radiation. This is the extreme limit of the heat the 

 sun could acquire. Professor Young says if this hy]jothesis 

 is true, as it probably is in the main, we are inexorably shut 

 up to the conclusion that the total life of the solar system 

 from its birth to its death is included in some such period as 

 thirty millions of years. No reasonable allowances for the 

 fall of meteoric matter based on what we are now able to 

 observe, or for- the development of heat by liquefaction, 

 solidification, and chemical combination of dissociated 

 vapours could raise it to sixty millions. 



According to Geikie the argument from geological evidence 

 is strongly in favour of an interval of probably not much less 

 than one hundred millions of years since the earliest forms of 

 life appeared upon the earth, and the oldest stratified rocks 

 began to be laid down. 



Let us start on the assumption that the space in which the 

 members of the solar system now move was filled with the 

 matter now forming them. First, as to the constitution of 

 this nebula. Wo may either assume it to have been one 

 element out of which all substances have been in some way 

 evolved, or we may assume it to have been a mixture of 

 various substances which are themselves elements. Since 

 we have absolutely no proof that the substances known 

 to us as elements can be resolved into a singly element, we 

 are hardly justified in making the first assumption. Let us 

 then suppose the nebula to have been composed of diflferent 

 elementary sub.stances, which may have been present in 

 various quantities, and that these substances were uniforml}' 

 distributed throughout it ft'om centre to surface. Second, as 

 to its condition, we may suppose it to have been in a more 

 or less heated condition or at the absolute zero of 

 temperature. We have already seen that on the assumption 

 that it Avas in a heated condition we cannot account for such 



