Capital may consist of anything that can enhance a person's power to perform economically useful work. Investors use cash to generate income by investing in assets.  An investor entrusts someone else with their principal, in hopes of it yielding profits. In essence you are putting money in their vest pocket (investment), so that assets will come back in your pocket. 
From"deadly sin, capital offense, and to forfeit life. Principalis, first in importance, money on which interest is paid;"