Sentence:  [...] I have observ'd, that putting <WSD>fair</WSD> Water (whether Rain-water or Pump-water, or "May-dew", or Snow-water, it was almost all one) I have often observ'd, I say, that this Water would, with a little standing, tarnish and cover all about the sides of the Glass that lay under water, with a lovely green [...] 
Meaning of the word marked with <WSD> tags:  Unblemished (figuratively or literally); clean and pure; innocent.