Mackay makes it sound as if alchemists were more successful as time went on. Either because people's hopes were raised to a fever pitch, or the art of deception became so refined.
Stories about people like Jean Delisle, a French alchemist from seventeenth century Provence, are a great example of both possibilities in action.
Having already detailed alchemists' use of false-bottomed crucibles or wands filled with gold and stopped up with wax or butter, Mackay suggests that these were Delisle's tools of choice. And he insists that the man who was quite famous in his own day (having allegedly transmuted base metals into gold and silver hundreds of times in public places) was nothing more than a master of sleight of hand.
This stance makes a great deal of sense since Delisle refused to perform his transmutation before the king thrice. In fact, Delisle had to be drug to Paris. Even then he never performed before the king based on the excuse that he couldn't in fact make the Philosopher's Stone, but an Italian had given him what he had used before and had exhausted his reserves of it during his years in Provence.
Of course, being from a period of time closer to Mackay's, it's no surprise that so much more information about Delisle is available to him and thus he treats him with much more nuance than the earlier practitioners of alchemy.
It's also to be expected that being alive in the midst of the Victorian era, with its legions of genre writers (then largely called "hacks") and poets, Mackay points out the literary debt owed to the Rosicrucians' benevolent spirits (sylphs, undines, gnomes, salamanders) from their emergence in the seventeenth century up to his own day. Such creatures remain a steady inspiration even to today's fantasy writers and figure largely into the mythos of the incredible Tales of series of JRPGs.