160 Sir Robert Ball [May 16, 



the Director of the Yerkes Observatory, and Professor Campbell, the 

 Director of the Lick Observatory, and I have also pictures taken by 

 Dr. Isaac Eoberts and by Dr. W. E. Wilson. These plates suggest 

 the wonderful variety and abundance of the nebulous contents of the 

 heavens, and throw much light on the Nebular Theory of the solar 

 system. It seems to follow from the researches of the late lamented 

 Professor Keeler that enormous numbers of spiral nebula) lie within 

 the reach of our photographic plates. Indeed it is not too much to 

 say that next to a fixed star itself, the spiral nebula is the most 

 characteristic object in the heavens. The significance of this state- 

 ment in connection with the Nebular Theory can hardly be over- 

 estimated. There can be little doubt that at one stage of the history 

 of the solar system the gradually evolving nebula must have formed 

 an object of that type which we term spiral. 



There is also another most remarkable discovery of modern times 

 which has added much weight to the arguments in favour of the 

 Nebular Theory. If the Sun and the Earth — to confine our attention 

 solely to those two bodies — had originated from the primaeval nebula, 

 they would bear with them, as a mark of their common origin, a 

 striking identity in material and composition. We do not of course 

 mean that the nebula was homogeneous all through, Nature does not 

 like homogeneity. The nebula was evidently irregular, vague in 

 form, dense in some places, greatly rarefied in others. We by no 

 means assert that if we compared a sample of the nebula in one 

 place with a sample of the same nebula taken a hundred or a thou- 

 sand million miles away from it, that the two samples would show 

 identity of chemical composition. We need not be surprised at this, 

 remembering that two samples of rock from the same quarry would 

 not be identical. But we may feel confident that the elements present 

 in the nebula will be more or less widely dispersed through it, so 

 that if two globes are formed by concentration in different parts of 

 the nebula, we might reasonably expect that though these two globes 

 would not be actually identical yet that the elementary bodies which 

 entered into their composition would be in substantial agreement. 

 If one element, say iron, was abundant in one body, we should reason- 

 ably expect that the same element would not be absent from the other. 

 Lai)lace had no means of testing this surmise, but our modern methods 

 enable us to investigate the chemistry of the Sun, and have shown 

 that the elements of which the Sun is composed are practically the 

 same elements as those of which our Earth is built. Is not this 

 a weighty piece of testimony in favour of Laplace's theory ? 



Laplace knew not of these photographic and spectroscopic revela- 

 tions ; he based his belief in the Nebular Theory mainly on a remark- 

 able deduction from the theory of probabilities. If the evidence thus 

 derived seemed satisfactory to Laplace one hundred years ago, this 

 same line of evidence, strengthened as it has been by recent dis- 

 coveries, is enormously more weighty now. 



Laplace was able to count up about thirty instances in which 



