ISO Rev. W.V. Harcourt on Lord Brougham's statements 



are intermingled with this subtle matter. We mean by air the 

 aggregate of both these, or whatever else makes up that hete- 

 rogeneous fluid wherein we breathe, commonly called air, the 

 purer part of which is Mr. Hobbs's air, and the feculent of it 

 is M. Hugens's air*." 



It is curious to trace the fortunes of this materia subtilis, 

 from the naked condition in which it was first ushered into 

 notice, to the figure which it now makes in the speculations of 

 science. 



Descartes was undoubtedly the first who formed the idea of 

 a liquid medium grosser than heat, but more subtle than air, 

 extending from the heavenly bodies to the earth, filling the 

 aerial interstices with a continuous series of molecular globules, 

 pervading the pores of glass, diamond, and the densest sub- 

 stances, without interruption, and propagating, by communi- 

 cation of impulses from one molecule to another, the move- 

 ment, or rather the pressure without locomotion, simple and 

 compound, which he considered as constituting lightf and 

 colours. 



This was a grand conception, for which the philosophy of 

 optics is under an obligation to the inventor greater perhaps 

 than has been confessed. But the range of Descartes's views 

 in physics was too limited to admit of his turning such a con- 

 ception to its full account. He seems to have had no idea of 

 intermittent or elastic forces, and did not even endow either 



• Extract of Letters from Dr. J. Wallis to the publisher, 1672, Phil. 

 Trans. No. 91. 



f Dr. Whewell takes Descartes's " hypothesis concerning light " to have 

 been, " that it consists of small particles emitted by the luminous body," and 

 considers this as " the first form of the emission theory " (Phys. Optics, 

 eh. x.) ; and so the theory of the Dioptrics seems to have been understood 

 by some of Descartes's cotemporaries ; but he explains himself otherwise in 

 his letters. " Je vous prie tie considerer que ces petits globes dont j'ai 

 parle nesont point des corps qui exhalent et qui s'ecoulentdesastresjusques 

 a nous ; mais que ce sont des parcelles imperceptibles de cette matiere 

 que V. R. appelle elle-meme celeste, qui occupent tous les intervalles que 

 les parties des corps transparents laissent entre ellcs, et qui ne sont autre- 

 ment appuyees les unes sur les autres que le vin de cette cuve que j'ai 

 pris pour exemple en la page 6 de ma Dioptrique, ou Ton pent voir que le 

 vin qui est en C tend vers B, et qu'il n'empeche point pour cela que celui 

 qui est en E ne tend vers A, et que chacune de ces parties tend a descendre 

 vers plusienrs divers endroits, quoiqu'elle ne se puisse mouvoir que vers un 

 6eul en meme temps. Or j'ai souvent averti que par la lumiere je n'entendois 

 pas tant le mouvement, que cette inclination ou propension que ces petits 

 corps ont a se mouvoir; et que ce que je disoisdu mouvement, pour etre plus 

 aisement entendu, se devoit rapporter a cette propension ; d'oii il est mani- 

 feste que, selon moi, Ton ne doit entendre autre chose par les couleurs que 

 les differentes varittes qui arrivent en ces propensions." (CEuvres, torn. vii. 

 p. 193> "J' admire que vous alleguiez les pages 4 et 5 afin de prouver que 



