The Constitution of Matter 73 



A further characteristic of liquid bodies which serves 

 to distinguish them from solids is the fact that a given 

 mass of liquid requires lateral support, i.e. it must be 

 put into a vessel with sides sufficiently strong to support 

 its weight. Unless thus supported, the liquid spreads 

 itself under the influence of the force of gravity as 

 a thin layer over the surface of whatever solid body is 

 situated immediately beneath it. This point of dis- 

 tinction between solids and liquids is sometimes 

 expressed by the statement that the smallest particles 

 or molecules (see later portion of this chapter) of which 

 a solid body is composed possess the property of 

 cohering together firmly enough to afford each other 

 mutual support, while, in the case of the molecules 

 of a liquid, the cohesion between them is not sufficient 

 to bring about this result. 



43. A gas resembles a liquid in many of its pro- 

 perties, but has no free surface. Any quantity of a gas, 

 however small, when introduced into a previously 

 empty vessel, immediately extends in all directions to 

 the furthest limits of the vessel, and a sample of the 

 contents, taken from any region within the vessel, 

 will be found to give the characteristic reactions of the 

 gas introduced. Further, if the vessel be transparent, no 

 free surface may be seen marking off the limit of the 

 gas, as is the case with a liquid. A gaseous body, in 

 common with all others, does, however, possess a 

 definite mass. With regard to change of shape, it is 

 a matter of common knowledge that a gas offers less 

 resistance to deformation than a liquid, e.g. it is easier to 

 move the hand backward and forward in air than in water. 



The considerable similarity in behaviour of liquids 

 and gases has led to their being classed together under 

 the general name of "fluids". 



