ATOMIC WEIGHTS. 51 



in these formulas or symbols? In the first place 

 they tell us at a glance of what elements any of 

 these substances are composed. But they do still 

 more. They, secondly, enable us to figure out the 

 exact proportion of any element in the compound. 

 For instance, we desire to discover how much 

 nitrogen there is in a given quantity of ammonia 

 (HaN). The compound consists of three atoms of 

 hydrogen and one of nitrogen. The atomic weight 

 of hydrogen was given as 1; the atomic weight of 

 nitrogen as 14. Thus we have 



3 H @ 1 -= 3 

 1 N @ 14 = 14 



Total, - - . 17 



The compound has an aggregate of 17 weight 

 units, of which nitrogen has 14. In other words: in 

 every 17 lbs. of ammonia we have 14 lbs. of nitrogen 

 and 3 lbs. of hydrogen. 



Another instance. We have a chemically pure 

 sample of nitrate of soda (Na NOs ), and wish to 

 figure out the percentage of nitrogen — which is the 

 element of value in the compound. 



The atomic weights are as follows: 



Na (one atom of sodium @ 23) - - - - 23 



N (one atom of nitrogen @ 14) 14 



O3 (three atoms of oxygen @ 16) - - - - 48 



Total, 85 



Thus in every 85 weights of the compound we 

 have 14 weights of nitrogen; in every 1 lb. of the 

 compound |f lb. nitrogen; in every 100 lbs. of the 

 former iff ^ lbs. or 16.47 per cent, of nitrogen. 



Still another example. We wish to find out what 



