THE CHEMICAL BASIS OF LIFE 233 



into steam or ice, the solution of sugar in water, the 

 mechanical movement of bodies, and the propagation 

 of sound, light and electricity are physical changes and 

 their treatment belongs to the science of physics. 



One striking peculiarity of chemical changes is that they 

 occur between certain definite proportions of the elements 

 involved. This principle is called "the law of definite 

 proportions." Thus sodium and chlorine always combine 

 in a certain definite ratio by weight, 23 parts of sodium 

 to 35.5 parts of chlorine to form common salt, or sodium 

 chloride. If more of one or the other element is present 

 it simply remains uncombined. Sometimes different 

 elements may combine with each other in more than one 

 ratio, but the different ratios have a simple relation to 

 one another. Thus carbon and oxygen may combine 

 in the ratio of twelve parts of carbon to sixteen parts 

 of oxygen to form a gas, carbon monoxide (CO), and also 

 in the ratio of twelve parts of carbon to thirty-two of 

 oxygen to form carbon dioxide or carbonic acid gas 

 (C0 2 ) which has properties very different from those of 

 carbon monoxide. In carbon dioxide there is just twice 

 as much oxygen in the compound as in carbon monoxide. 

 The definite numerical ratios in which elements unite 

 into compounds form one of the several considerations 

 that have led men of science to the view that chemical 

 elements are made up of minute, indivisible bodies called 

 atoms. The atoms may be united into groups called 

 molecules, a molecule being the smallest part into which 

 a compound may be divided without losing its properties. 

 The division of a substance into molecules may involve 

 nothing but physical changes, but to divide a molecule 

 into its constituent atoms would constitute a chemical 

 change. 



In order to express the chemical constitution of bodies 



