no 



we can put the colours all together and their union is white again. 

 We are able to separate it as distinct lights and possessing distinct 

 properties. By voltaic action, sparks like those of the electrical 

 machine may be produced which derive their characteristics in 

 great measure from the metal from which they spring ; and the 

 characters of those metals give different characteristics to the 

 sparks. We have one set of these produced by iron, another by 

 nickel, and others by hydrogen gas, and so on. These have been 

 observed and catalogued with great care. When we observe the 

 light of the stars in like manner, we find there are scarcely two 

 stars alike. Some are found exhibiting that light which is given in 

 sparks from iron, and others shewing the light derived from a 

 number of different things, and we are actually able by legitimate 

 reasoning to say what the stars are made of As a general rule 

 there are hardly two stars alike, so that in this nebular hypothesis 

 we are not bound to say that the nebula; are all of the same 

 materials ; and we may go further in this assertion when we come 

 to consider other matters, and say that even the parts of any 

 one nebula are not of the same materials ; and we are justified in 

 saying so, by comparing things that we know of the bodies of 

 the solar system. We cannot see anything different in the light 

 from them, because they all get their light from one source, that of 

 the sun. But it is in regard to what we know of the densities that 

 we are justified in saying that their compositions are different. As 

 I have said, the average density of the earth is probably five-and- 

 a-half times that of water. But the density of the sun is only once 

 that of water. The density of Mercury is nearly the same as that 

 of the earth ; the density of Venus and Mars is much the same. 

 Then comes a shower of little planets numbering about two 

 hundred, and we cannot tell what these are made of Jupiter and 

 Saturn are no heavier than water. Therefore assuming the form- 

 ation of these things by condensation of nebute, on the theory I 

 have mentioned, it appears clear that the different parts of nebulae 

 which have contributed to form our solar system are very different. 

 That being considered as established, it follows as likely enough 

 that in the constitution of our earth there are parts of very different 



