34 THE MECHANISM OF LIFE 



of molecules, each, of which contains a relatively small number of 

 one or more kinds of chemical atoms. 



Now colour, smell, taste, lustre, texture, temperature, and 

 density, are dependent on the chemical nature of the molecules, 

 ,and on the ways in which the latter are arranged and on their 

 motions. Thus water may be warm or cold to our sense, but it 

 consists in each case of the same molecules moving relatively 

 slowly when the water is cold, and relatively quickly when it is 

 warm. Phosphorus may be yellow or red, and in the former case 

 it is odoriferous, poisonous, and combustible; while in the latter 

 case it has no smell, is not poisonous, and is non-inflammable in 

 the conditions in which yellow phosphorus is inflammable. Yet 

 the substance phosphorus is chemically the same in both cases, 

 only the atoms are in different configurations. The same number 

 of molecules of HgO may be dense in the form of ice, less dense in 

 the form of water, and less dense still in the form of steam, accord- 

 ing to the distances that its molecules are apart from each other. 

 Something, however, is the same in the case of the red and yellow 

 phosphorus, or the solid, liquid, and gaseous water — that is, the 

 mass of the chemical substance itself. Apparently colour, taste, 

 smell, density, temperature, etc., may be variable, while mass 

 remains invariable. 



Weight and Mass. — Weight itself is something that may vary, 

 ' while mass remains the same. A material body weighs more at 

 the earth's poles than it does at the equator, and if it could be 

 transported to the sun it would be much heavier, or if to the 

 moon much lighter, than it is on the earth. If it could be removed 

 to several millions of miles away from any cosmic body its weight 

 would be almost nothing. "Weight depends on the mass of the 

 body, on its distance from some attracting body (such as the sun, 

 earth, or moon), and on the mass of the latter. The mass of our 

 material body would be the same everywhere (we are neglecting 

 some physical results included in the theory of relativity), but its 

 weight would vary. 



Mass. — Something, then, seems to be invariable, or nearly so, 

 and this is the mass or quantity of matter in a body. The mass 

 of a cubic inch of iron is so much in all circumstances, and that of 

 , a cubic inch of gold is more than that of the same bulk of iron, but 

 is also invariable. By the quantity of matter, or mass of a body, 

 we therefore mean the number of molecules in the body multi- 



