The Relativity of Knowledge. 39 



ated atoms, will help us transcend the difficulty. !N"o mat- 

 ter how continuous a body may seem to be to our senses, 

 chemistry and physics join hands in telling us that this is 

 a mere illusion. The most solid steel is built of molecules 

 that are not and cannot be in actual contact with each 

 other.* They exist in it like a cloud of gnats or flies, and 

 only appear one instead of many because they move to- 

 gether as a niass. If two pieces of steel resist each other, 

 and refuse passage-way the one to the other, it is not be- 

 cause there is not room enough for both pieces within the 

 same superficial limits, but because they are acting like two 

 belligerent flocks of birds, and meeting with their mole- 

 cules, breast to breast. How easy it would be for two 

 flocks of pigeons to pass each other in opposite directions, 

 without the least resistance, if they would only fly in inter- 

 mediate spaces. A distant onlooker could then see them 

 apparently occupying the same space at the same time. 

 The mass of a cubic inch of mercury is far greater than 

 that of a cubic inch of aluminum, and yet the resistance 

 of the latter is far greater than that of the former. If a 

 trip-hammer is adjusted in its rhythm to seconds, and your 

 finger can be waved to and fro beneath it in intermediate 

 time, no harm can befall you. A single maladjustment 

 will cause resistance, or the destruction of your finger. 

 Let the rhj'-thm of the molecules of this disk be adjusted to 

 the rhythm of those of my hand so that they can freely 

 pass each other, and practically the desk will have become 

 non-existent. Adjust those of our bodies to those of the wall, 

 and Ave will no longer need doors. The wall resists light 

 just as it does our bodies ; but glass that may be much denser 

 allows that same light to go through, practically unob- 

 structed. As the light goes through the glass, or electricity 

 through a copper wire, although neither can force a passage- 

 way through a brick, so we go through the solid ether, but 

 cannot pierce the less dense wall. Solidity is only a term 

 belonging to relations, and not an actual condition of being 

 in itself. For anything that we can know to the contrary 

 there may be, right here and now, passing through us and 

 this world, some planet invisible to us, with mountains, 

 oceans, lakes, rivers, cities and inhabitants.! To either 

 affirm or deny betrays our mental weakness. We are 



* Maxwell's Theorj- of Heat, pp. 281 -287. 



t Jevon's Principles of Science, Vol. 2, p. 145. Young's " Works," Vol. 1, p. 417. 



