76 DEFINITE PROPOBTIONS. 



588 grains of sulphuric acid. The composition of 

 100 grains of such a substance is represented thus : — 



Lime 41.2 



Sulphuric Acid . . . . . 58.8 



This merely means that one hundred grains contain 

 41 and two-tenths of a grain of lime, and 58 and eight- 

 tenths of a grain of sulphuric acid : hence there is no 

 real difference whether we put the dot or not : if it 

 is used, the figures behind are known to be fractions ; 

 if not they are whole grains. 



137. Sulphate of lime may be expressed either as 



1 grain. 10 grains. 100 grains. 1000 grains. 



In all the analyses given in the following pages, no 

 fractions are used ; the composition of everything is 

 given as it would be obtained if 10,000 or 100,000 

 grains were analyzed. 



138. Chemists are constantly in the habit of speak- 

 ing of atoms, proportions, combining numbers, and 

 similar terms ; it will be well briefly to describe what 

 is meant by these words. It has been already stated 

 that the composition of all compound substances is de- 

 finite (124); that a certain weight of carbon, for ex- 

 ample, can only combine with a fixed quantity of 

 oxygen, to form carbonic *cid (126). 



139. Let us observe what relation there exists be- 

 tween the quantity of difi"erent substances which can 



