The Constitution of Matter 81 



in proportion, so that the pressure of the atmosphere 

 on the surface of the water in the outer vessel, which 

 pressure remains practically unaltered throughout the 

 experiment, is able to overcome that on the water 

 inside the tube and thus forces some of the water from 

 the vessel into the tube. 



Exercises for Student. 



1. Try to discover the reason for covering the top of the tube 

 while it was being filled with coal-gas. 



2. Assuming the Kinetic Hypothesis, account for the gradual 

 fall of the water inside the tube after the maximum height reached 

 is attained. 



47. In the earlier part of this chapter, three different states 

 of matter, solid, liquid, and gaseous, were considered and their 

 more important properties dealt with. It must not be inferred 

 that all kinds of matter may be classified rigidly under these three 

 heads. In scientific work much classification is done, with a view 

 to rendering large numbers of individual facts seemingly isolated 

 easier to grasp, and many definitions are framed, in order to give 

 clearness and exactness to the various ideas concerned. The reader 

 must be warned, however, that, although the boundaries of classes 

 may be made as definite as possible and the definitions worded as 

 clearly as may be, it is rarely the case that no overlapping occurs in 

 classification or that a definition covers completely every example 

 that may come within its terms. Thus, although our arrangement 

 of bodies into the three groups above named is exceedingly con- 

 venient, any given body does not of necessity fall into one of those 

 groups. Putty and pitch would probably be classified as solids 

 from a cursory examination, since one can handle lumps of these 

 substances without requiring a containing vessel. Yet, when left 

 in a vessel for a sufficient length of time, they generally assume 

 the shape of its interior and present a horizontal free surface, in 

 this behaviour resembling liquids. Again, an experiment may be 

 arranged in which a liquid is heated in a stout glass tube under 

 pressure. At a certain temperature and under a certain pressure, 



H.D. s. 6 



