10 



molecules of chemical elements are composed of atoms of 

 the same kind. Compound substances have atoms of dif- 

 ferent kinds. There are as many khjds of atoms as of 

 elements. Both atoms and molecules are too small to be 

 seen even with powerful microscopes. 



A drop of water may be divided mechanically into 

 many small portions, and each part will retain all the 

 characteristics of the original drop. But when the chem- 

 ist separates the oxygen and hydrogen which together 

 make up the drop of water, he has no longer any matter 

 which resembles water, but instead two kinds of gaseous 

 matter of entirely different properties. The water has 

 been resolved by chemical analysis into its chemical 

 elements. 



Chemical elements unite in different proportions with 

 one another to form a great variety of substances. 

 About seventy-five elements have been isolated by chem- 

 ists, but only a few of them are known to enter into the 

 structure of animal bodies. These are carbon, oxygen, 

 hydrogen, nitrogen, sulphur, phosphorus, chlorine, fluorine, 

 r ; Silicon, potja^sjufmj sodium, lithium, calcium, magnesium, 

 'iro'n, afi(i ntahga'tfese. As a rule these elements exist in 

 , : *$e;Ucftty',i^ sofm?e;spri of combination with one another. 



'3. * Protoplasm is a name given to living matter. It is 

 a clear, jellylike substance containing minute grains. 

 As protoplasm, it cannot be chemically analyzed, because 

 the process of analysis destroys its vitality so that it is no 

 longer protoplasm, but merely dead, lifeless matter. The 

 one essential thing about protoplasm is that it is alive; 

 dead protoplasm is a contradiction in terms. It has been 

 called " the physical basis of life," because without it life 

 does not exist, and with it there is always life. But the 

 material of which protoplasm is composed is found, when 



