8 Elementary Biology. 



and others intermediate between any two of them. 1 It 

 is a familiarly known fact, moreover, that the same kind 

 of matter may exist in different states, under different 

 atmospheric and other conditions i.e. we may have trans- 

 formation of matter from one state into another.. Water is 

 probably the most familiar instance of this. Fluid under 

 ordinary temperatures, it becomes solid when the tempera- 

 ture is sufficiently reduced, or gaseous when sufficiently 

 augmented. Matter in its manifold state possesses the 

 same fundamental properties. 



Matter is divisible, but not infinitely so (save in imagina- 

 tion). Practically, and even theoretically, there are limits 

 to the divisibility of matter. The practical limits to the divi- 

 sibility of matter are soon reached ; but, theoretically, matter 

 is believed to consist of extremely minute and perfectly 

 invisible particles, or molecules. Matter is, therefore, not 

 absolutely continuous ; the molecules of which it is com- 

 posed are an appreciable distance apart, i.e. matter is porous, 

 and therefore penetrable or permeable. For it is conceiv- 

 able that smaller molecules of another substance might 

 be insinuated between the larger molecules. Molecules 

 themselves are, however, supposed to be impenetrable. 



The number of molecules in a given volume of a sub- 

 stance varies according as the substance is in the solid, 

 liquid, or gaseous state. Observation and experiment teach 

 us that the molecules are relatively close to one another in 

 the solid and liquid states, and that they are further apart 

 in the gaseous. 



Defined physically, a molecule is the smallest particle 

 of a substance that can possess the properties of the whole; 



1 No reference is made in the text to the recent interesting specula- 

 tion of Crookes and others on the possible existence of a fourth state 

 which matter is believed to be capable of subsisting in, when the tension 

 of its gaseous state is reduced to one-millionth of an atmosphere, as being 

 out of place in an elementary sketch like the present. See ' Radiant 

 Matter,' by W. Crookes, F.R.S., British Association Report^ 1879. 



