2 Preliminary Remarks. 



To show how this property, together with that of extension.. 

 is necessary to constitute a body, I will refer to familiar phenom- 

 ena in which these properties are observed separately. 



If an object be placed before a concave mirror, there will be 

 formed, at a certain distance from the mirror, an image of the 

 object. This image, distinct from the parts of the space that 

 surround it, is extended but not impenetrable. The hand may 

 be thrust through it without experiencing the smallest resistance, 

 and the parts that come in contact with the hand, vanish instead 

 of being displaced. A piece of wood or stone does not admit of 

 being thus penetrated. Moreover, by means of a second mirror 

 properly disposed, an image of another object may be made to oc- 

 cupy the same place with that of the first, without the latter being 

 displaced, or in any way deranged. Indeed the same coincidence 

 may be effected with a third, a fourth, or any number of images. 

 These images are extended, but not impenetrable ; they are/orras, 

 but not sensible matter. I say sensible matter, for we shall see here- 

 after that light which constitutes these images, is itself probably 

 composed of material particles of an insensible tenuity, which 

 move with amazing velocity, and only pass by each other in this 

 case at immense intervals, by which they are separated from 

 each other. 



4. It is here proper to speak of certain phenomena which 

 seem, at first sight, to be opposed to what we have laid down 

 with regard to the impenetrability of matter, but which, ex- 

 amined more attentively, only tend to confirm it. 



When a solid body is suffered to fall into any fluid, as water for 

 example, it sinks and seems to penetrate the fluid ; but it in fact 

 only separates and displaces the parts that compose it, for if the 

 vessel containing the fluid be formed with a narrow neck toward 

 the top, like a bottle, the fluid will be seen to rise as the 

 body enters, and to a greater or less height, exactly in propor- 

 tion to the size of the immersed body. What has taken place 

 therefore, is only a division and separation of parts, and not 

 strictly a penetration. The same may be said when an edged 

 tool is forced into a block of wood, only the parts of the wood 

 are separated with more difficulty than those of water. The same 

 may be said also when a nail is driven into clay, lead, or gold, 

 in which cases it only makes an opening sufficient for its admis- 

 Indeed the mass thus pierced is not entirely separated, 



