From Mackay's collection of biographies it sounds like alchemists took a turn for the worst in the Late Middle Ages. From the late fourteenth through the fifteenth centuries in particular, the alchemists that he describes are truly deserving of his scorn. Specifically the child-murdering Gilles de Rais and coin debaser Jacques Cœur.
Their crimes aren't exactly on par with each other, but both are terrible reprobates that hid their wrong doings behind a screen of being alchemists searching for the philosopher's stone.
And this apparent state of things makes me wonder.
In the case of Roger Bacon, his grand intellect drew claims and accusations of sorcery. His contemporaries (and many later generations) regarded such claims as a great blow to his reputation. Currently though, and even in Mackay's day, people are much more able to regard Bacon's discoveries as quietly revolutionary, at least in so far as they ran far ahead of what science was doing in his time.
Now, turning to the groups of alchemists motivated by greed and a lust for power, covering themselves with the name "alchemist" (or, more accurately, "alchymist"), it's interesting how the same thing happened.
Most modern readers and Victorian skeptics like Mackay aren't fooled by such titles. Instead, they're able to see past them and to the bearer's crimes. Yet, it's not that the label "alchemist" has many negative connotations, it's that it has hardly any now. At least in the sense that anyone claiming to be an alchemist these days is likely to be dismissed out of hand (and the same could be said for someone claiming to be a "sorcerer").
And that's just what happened in both cases.
The negative label didn't turn into a positive one. Rather, it lost its adhesion and just fell off, revealing the terrible stain that it had been covering over.