40 CHEMISTRY. 



36. Mathematical Calculations. You have already obtained some 

 insight into the numerical relations of the atoms ; the calculations made 

 from the atomic weights and molecular weights of bodies belong to a special 

 branch of chemical philosophy called stoichiometry, a word made up of two 

 Greek words meaning "an element" and " a measure." These mathemat- 

 ical calculations are of the greatest importance both to the scientific and 

 manufacturing chemist ; the former is enabled to determine the composi- 

 tion of new bodies, or the purity of known substances ; the manufacturer is 

 enabled to estimate how much of any given material it is necessary to use 

 in order to manufacture a certain product. A knowledge of a few simple 

 rules, seldom requiring higher arithmetic than Proportion, properly ap- 

 plied to questions of practical import in manufacturing chemical products, 

 saves the capitalist thousands of dollars, and dollars and cents are items of 

 no small consideration. We can not explain all these calculations to you, 

 nor is it necessary for any one except a working chemist to master them, 

 but you should endeavor to realize their importance. We will give only 

 a single case, and illustrate it by two examples. Turn to page 38 ; you 

 find there an equation for the reaction of potassium in water, with the nu- 

 merical values of each atom and molecule attached. Now suppose this was 

 a good method for manufacturing potassium hydrate (or caustic potash, as 

 it is commercially known), and we should want to know how much of it 

 can be made from 10 pounds of metallic potassium, the question would be 

 solved thus : 



From the equation cited it is evident that one atom of potassium yields 

 one molecule of potassium hydrate ; but one atom, or K, weighs 39.1, and 

 the molecule KHO weighs 56.1, therefore we have the proportion : 



Atomic oecuar The number 



39.1 : 56.1 :: 10 : the answer; 



56 1x10 

 whence ' ~-. =14.3; consequently 10 pounds of potassium would 



oJ. 1 



furnish 14.3 pounds of potassium hydrate. 



Supposing, again, that this was an economical method for manufactur- 

 ing hydrogen gas, how much potassium is required to make 100 grammes 

 of hydrogen ? By examining the equation on page 38, you will find as 

 before that one atom of potassium weighing 39. 1 takes place in the re- 

 action setting free one atom of hydrogen weighing 1 , hence the propor- 

 tion : 



