ii The Substance of Nature 23 



rigidity is of course greater for greater masses, and the 

 amount of the pull in any case may be measured by 

 pulling against it. Weight is the name given to the pull 

 of the Earth upon some other body. At any definite 

 distance from the Earth's centre the weight of a body is 

 proportional to its mass, and hence it is that when we want 

 one pound mass of tea we ask for one pound weight. If 

 any mass is removed to a greater distance from the Earth's 

 centre the pull upon it is diminished, or, in other words, its 

 weight is less ; if it is brought nearer the centre (without 

 passing inside the Earth) the pull upon it is increased, 

 or the weight is greater. Weight, or " Earth-pull," is 

 measured by means -of the spring -balance or by the 

 pendulum. On account of the uniform pull of the Earth's 

 gravitation, liquids, which have no rigidity, assume a level 

 surface, or rather a surface parallel to that of the Earth. One 

 of the necessary conditions for equilibrium in a liquid is that 

 all points in the same plane are subject to the same pressure, 

 hence the level of water in a series of connected vessels is 

 always the same. Hence also if the height or the density 

 of a column of liquid is altered equilibrium is destroyed, 

 and the liquid moves under the influence of gravity until 

 it again becomes homogeneous and of level surface (see 

 238). Gravitation is a property which affects every kind 

 of matter alike, and it binds together the great masses of 

 the Universe into a firm and flexible whole. 



39. Cohesion. When the distance between particles of 

 matter is very minute too small to be measured the force 

 of attraction is very great, and binds the particles together 

 very firmly. In this case it is called cohesion. It is by the 

 powerful attraction of particles of matter at very minute 

 distances that a stone is wetted or covered with a thin liquid 

 film when dipped in water. These forces are also shown at 

 work when a liquid rises in a narrow tube, or in a porous 

 body like a sponge, a lump of sugar, or a piece of sand- 

 stone. This raising of liquids is called capillarity because 

 it is best seen in tubes whose bore will just admit a 

 hair, but it is quite visible on the sides of a tumbler. 

 Another manifestation of the same force is seen in surface 



