ii The Substance of Nature 17 



27. Matter is that which occupies space. This defini- 

 tion is in many ways the most satisfactory ; but although 

 attempts to say what matter is have been made by philo- 

 sophers in all ages, no really sufficient definition has ever 

 been arrived at. Matter is often defined as that which can 

 be perceived by the senses. 



28. Mass is the term used to denote quantity of matter. 

 Thus when the mass of the Sun is spoken of as being 

 300,000 times that of the Earth, it is meant that the Sun 

 contains 300,000 times as much matter as the Earth con- 

 tains. Mass is usually measured out by the balance, 

 and it is common to speak of the mass of any portion of 

 matter as its weight ( 38), although on the same principle 

 we might speak of a man's health as his appetite. The 

 unit of mass in British Possessions and the United States 

 is the pound ; in almost all other civilised nations it is the 

 kilogram. (See 437.) 



29. Volume and Density. Volume is the amount of 

 space occupied by a body, and if matter were of one kind and 

 always in the same state, the same mass would always fill 

 the same volume. But matter exists in many forms, and 

 if, for example, we compare together charcoal, lithium, coal, 

 granite, arsenic, lead, and platinum, we find that the same 

 volume contains very different quantities of matter. Indeed, 

 the mass of a cubic inch of platinum is twice that of a cubic 

 inch of lead, four times that of arsenic, eight times that of 

 granite, sixteen times that of coal, thirty-two times that of 

 lithium, and sixty-four times that of charcoal. So that these 

 parcels of matter are packed with different degrees of 

 tightness, as much as is present in 64 -cubic inches of 

 charcoal being packed within the limits of I cubic inch of 

 platinum. The amount of matter in a unit of volume is 

 called its density ; thus in the list given above the density 

 of each substance mentioned is twice that of the preceding. 

 The unit of density universally employed is that of water, 

 and calling this I the densities given above run : 



Charcoal. Lithium. Coal. Granite. Arsenic. Lead. Platinum. 



0-34 0-59 1-33 2-70 5-96 11-36 21-53 



