MEASUREMENT OF DENSITY 45 



The meaning of density 1 . Pieces of different substances of the 

 same size or volume may have unequal weights. 



2. Pieces of different substances which have equal weights may have 

 very different sizes or volumes. 



It is usual to speak of these facts by saying that things have different 

 densities. A pound of feathers, or cotton- wool, has exactly the same 

 weight as a pound of lead, but the feathers (or the cotton-wool) take up 

 much more room, or have a larger volume than the piece of lead. The 

 matter in the lead may be packed more closely than in the feathers, 

 which accounts for its taking up less room. The shortest way of saying 

 all this is to say that lead is denser than either cotton-wool or feathers. 



If a small thing is comparatively heavy, then it is called a dense 

 thing, or it is said to have a high density. If, on the other hand, a 

 large thing has a small weight, it is said to have a low density. More- 

 over, the densities of things having equal volumes are in the same 

 proportion as their weights. 



Standard of density. But to compare densities it is better to have 

 a standard, just as there is a standard of length, the yard, with which 

 to compare other lengths ; or a standard of area with which to compare 

 other areas. The density of water at a certain fixed temperature is 

 taken as the standard. Why it is necessary to specify the temperature 

 will be understood later. 



The weight of one cubic centimetre of water at a temperature of 

 4 C. is one gram, and its density is taken as the standard of density, 

 expressed in grams per cub. cm. Similarly, a substance, the weight 

 of a cubic centimetre of which is two grams, would be said to have a 

 density of 2 grams per cub. cm., for it must contain twice as much 

 matter as water does, packed into one cubic centimetre. The weight 

 of a cubic centimetre of quicksilver is 13'6 grams, i.e. it contains 13'6 

 tunes as much matter in one cubic centimetre as there is in one cubic 

 centimetre of water. Its density is therefore 13'6 grams per cub. cm. 



Density is the weight of unit volume of a substance. Suppose 

 a cube of soap and a cube of lead be cut so as to have equal weights ; 

 the soap will evidently be larger than the lead, and it will be just as 

 many times larger as its density is less than the density of the lead. 

 With equal weights, the greater the density the smaller is the volume. 

 It follows from this that if the volume of a body is multiplied by its 

 density, we obtain its weight. Or expressed as dn equation, 



volume x density = weight. 

 From this it follows that weight 



volume* 



