ROYGBIV is an <em>acronymic abbreviation</em> because this word contains a set of initial letters which stand for the following words: Red, Orange, Yellow, Green, Blue, Indigo and Violet. And all these words represent the order of colors in a rainbow.
Though I can't see all the answers here's my take:
The author was making a point that Voltaire's humor was not welcomed because it criticised their beliefs and the book was banned because it was full of his criticising humor that they did not appreciate.
Chemical formula tells us the number of atoms of each element in a compound. It contains the symbols of the atoms of the elements present in the compound as well as how many there are for each element in the form of subscripts.
