Marshall, Alfred. <br/><br/>
Principles of Economics /  

 - Ninth Ed. /  
 - London :   Macmillan and Company Ltd.. 
 - p885 :  
<br/><br/><label>Subjects--Topical Terms: </label><br/>Economics 
<br/><br/><label>Dewey Class. No.: </label>X  / C0,K1.2