Light, Water, Hydrogen

In addition to domestic animals the earliest records of mankind indicate that slavery, until the use of coal became widespread, has always been a significant aspect, or part, of nearly every society. Consider for example ancient Attica (Greece), in which 115,000 out of a total population of 315,000 were slaves [1]. For the lucky rulers slaves represented power, Joule/second or Watt. On a steady st...