Human Physiology. 99 



water drawn nearer together with the diminution of heat, at a 

 certain coldness again fly apart, so that ice forms on the surface, 

 .and protects the water and its inhabitants below. But for this 

 all lakes and streams in cold countries would become solid ice 

 and so remain. Ice is water crystallised, and the expansive 

 force of this crystallisation is so great that it will burst the 

 heaviest cannon in which water is closely confined. May not 

 the crystallisation of harder bodies be accompanied with force 

 enough to upheave mountain ranges and whole continents? 



A form of cohesive force or attraction is that called capillary. 

 Oil rises in the small interstices of a cotton wick. Water 

 rushes eagerly up the pores of a sponge or lump of sugar. It 

 stands at a certain height above the general surface in a small 

 tube, and the smaller the tube the higher it mounts, apparently 

 by the cohesive attraction of the sides of the tube. Plants and 

 trees are full of microscopic tubes in which the sap ascends 

 from the roots to the most distant branches even if three 

 hundred feet high. In animals, fluids circulate through an 

 immense number of hair- like tubes ; moving forward where no 

 force can propel, and no attraction but this of the vessels 

 themselves can draw. Liquids also pass through the pores of 

 vegetable or animal membrane with sufficient force to sustain a 

 column of water in a tube several feet high, or burst closed 

 membranous sacs with considerable violence. 



The force of electricity is equally wonderful and incompre- 

 hensible. I pass a vulcanite or hard india-rubber comb 

 through my hair, and on holding my knuckle near it, draw 

 from it a spark of fire with a corresponding explosion. I rub a 

 stick of sealing-wax or a glass tube on my coat sleeve, and it 

 will draw and then repel bits of paper, or give off a spark. A 

 cloud approaches the earth suddenly falls a bolt of fire as 

 large as a hogshead a blinding flash an explosion that 

 shakes the earth, and I find a pine tree shivered into a 

 thousand fragments. Faraday says that the electricity set free 



