THE VISIBLE IS ONLY ITS SHADOW. 



59 



hand, perception by the sense of touch tells you that it is con- 

 tinuous or what is called solid and hard ; but it is not so in reality 

 except as a concept limited by our finite senses. A fair analogy 

 would be to liken it to a swarm of bees, for we know that it is 

 composed of an immense number of independent atoms or mole- 

 cules, which are darting about and circling round each other at 

 an enormous speed but never touching ; they are also vibrating 

 at a definite enormous rate which we can, at will, increase b} 

 heat or reduce by cold. If we heat the cannon-ball we increase 

 this vibration so that the cohesion of the atoms decreases so 

 much that the iron becomes liquid, and further heating produces 

 volatilization, where tKe property of what we call solidity dis- 

 appears. Let this vapour now be cooled and, passing again 

 through a fluid state, the cannon-ball takes on the appearance of 

 solidity. If our touch perception were sensitive enough we should 

 feel the vibration of the atoms and should not have the sensation 

 of a solid. We have a similar case of limitation in our other 

 senses. As I have already shown you, we can hear pulsations 

 only up to fifteen in a second, beyond that number they give the 

 sensation of a musical or continuous sound. In our sense of 

 sight we can see pulsations or intermittent flashes up to only six 

 in a second, beyond that number they give the sensation of a 

 continuous light; a gas jet, if extinguished and re-lit six times a 

 second, can be seen to flicker, but beyond that rate it is to our 

 sense of sight a steady flame; the same effect may be shown by 

 the red glow of the top of a match ; when stationary or moving 

 slowly it is a point of light, but, moved quickly, it becomes a con- 

 tinuous line of light. Even apart from our senses we find motion 

 giving the characteristics of solidity. A wheel, with only a few 

 thin spokes, if rotated quickly enough, becomes quite impene- 

 trable by any substance, however small, thrown at it. A thin 

 jet of water only half an inch in diameter, if discharged at great 

 pressure, equivalent to a column of water of 500 metres, cannot 

 be cut through even with an axe, it resists as though it were made 

 of the hardest steel. A thin cord hanging from a vertical axis, 

 and being revolved very quickly, becomes rigid, and if struck 

 with a hammer, it resists and resounds like a rod of wood. A 

 thin chain and even a loop of string, if revolved at a great speed 

 over a vertical pulley, becomes rigid and, if allowed to escape from 

 the pulley, will run along the ground as a hoop. 



Again we appear to have no sense of direction when travelling 

 through space, except by noting passing objects. If we are in 

 a train with the blinds down we cannot tell in which direction 

 we are going, and even if we have that knowledge, and the train 

 by going in and out of a terminus has, without our knowledge, 

 changed its engine so that we, without moving, are occu])ying 

 a back instead of a front seat, we are not conscious of this 



