1881.] from the time of Newton. 139 



amount, or even a thousandth part of it, could have existed at any 

 one time in our terrestrial atmosphere since the beginning of life 

 on our planet is inconceivable, and that it could be supplied from 

 the earth's interior is an hypothesis equally untenable. 



I was therefore led to admit for it an extra-terrestrial source, 

 and to maintain that the carbonic acid has thence gradually come 

 into our atmosphere to supply the deficiencies created by chemical 

 processes at the earth's surface. Since similar processes are even 

 now removing from our atmosphere this indispensable element, and 

 fixing it in solid forms, it follows that except volcanic agency, 

 which can only restore a portion of what was primarily derived 

 from the atmosphere, there are on earth, besides organic decay, only 

 the artificial processes of human industry which can furnish car- 

 bonic acid ; so that but for a supply of this gas from the inter- 

 stellary spaces now, as in the past, vegetation, and consequently 

 animal life itself, would fail and perish from the earth for want of 

 this " food of planets." 



Such were the conclusions, based on an induction from the facts 

 of modern chemistry and geology, which I enunciated in my paper 

 in 1878 and 1880, already quoted in the first part of this essay. I 

 was at that time unacquainted with the Hypothesis of Newton, and 

 with his remarkable reasoning contained in the 41st proposition of 

 the third book of the Principia, in which he, so far as was possible 

 with the chemical knowledge of his time, anticipated my own argu- 

 ment, and showed how and in what manner the interstellary ether 

 may really afford the " food of planets," and, in a sense, " the 

 material principle of life." 



I have thus endeavoured to bring before the Philosophical 

 Society of Cambridge, a brief history of the development of this 

 conception of an interstellary medium, and to show that the 

 thought of two centuries has done little more than confirm the 

 almost forgotten views of Newton. It is with feelings of peculiar 

 gratification that I have been able to indite these pages within the 

 very walls of the College in which our great philosopher lived and 

 laboured, and where, combining all the science of his time with 

 a foresight which seems well-nigh divine, he was enabled, in 

 the words of the poet, " to think again the great thought of the 

 Creation." 



