ON Tin ESSENTIAL PROPERTIES OT MATTER. 609 



visibility of matter with respect to coloured substances; for the colours of 

 thin transparent substances, which he considers as resembling those of most 

 other substances, are no longer observable, in any known medium, when the 

 thickness is less than about -ro-s-'o^-s- of an inch. But we have positive evi- 

 dence that coloured substances may be reduced to dimensions far below this 

 limit; besides the instance of the gilt wire, which has already been mentioned, 

 a particle of carmine may still retain its colour, when its thickness is no more 

 than one thirty milliontli of an inch, or one sixtieth part of the limit deduced 

 from the supposition of Newton ; and it is tlierefore scarcely possible that the 

 colours of such substances can precisely resemble those of thin plates, although 

 they may perhaps still be in some measure analogous to them. 



Impenetrability is usually attributed to matter, from the common observa- 

 tion that two bodies cannot occupy the same place at once. And it is thus 

 that we distinguish matter from space; for example, when we dip an in- 

 verted jar into mercury, the air contained in the jar depresses the surface of 

 the mercury, and prevents its occupying the space within the jar: but if the 

 jar had been void of matter, like the space above the mercury of a barometer, 

 nothing would have prevented its being filled by the mercury, as soon as 

 either its weight, or the pressure of the atmosphere, urged it to enter 

 the jar. 



But it does not appear that our senses are fully competent to extend this 

 proposition to all substances, whether material or not. We cannot prove 

 experimentally that the influence of gravitation; is incapable of pervading even 

 the ultimate particles of solid ,matter, for this power appears to suffer no di- 

 minution nor modification, when a third body is interposed between the two 

 gravitating masses. In the same manner, a magnet operates as rapidly on a 

 needle, through a plate of glass or of gold, whatever its thickness may be, as 

 if a vacuum only intervened. It may, however, be inquired if the gold or 

 the glass has riot certain passages or pores, through which the influence may 

 be transmitted : and it may be shown, in many instances, that substances, ap- 

 parently solid, have abundant orifices into which other substances may enter ; 

 thus mercury may easily be made to pass through leather, or through wood, 

 by the pressure of the atmosphere, or by any other equal foxce : and, how- 

 ever great we may suppose the proportion of the pores to the solid matter, it 



