Final answer:
The vulgar idea suggested that the fire in the scientist's laboratory was brought from the lower regions and fed with infernal fuel, reflecting a mystical or fearful perception of the scientist's work.
Step-by-step explanation:
The source of the fire in his laboratory according to the vulgar idea was that it had been brought from the lower regions, and was fed with infernal fuel. This description conveys a sense of otherworldliness and perhaps even a connection to the supernatural, suggesting that the people held a somewhat fearful and mystical view of the scientist's work. The choice of words like 'vulgar idea' and 'infernal fuel' underscore the general public's perception of the laboratory as a place of dark and ominous undertakings.