Final answer:
The alchemist's observation about the Sahara symbolizes the alchemical theme of transformation, paralleling the alchemical pursuit of converting base materials into noble ones.
Step-by-step explanation:
The old alchemist’s observations that the Sahara used to be a sea best conveys the alchemical theme of transformation and the quest for understanding the processes of nature. Alchemists were often concerned with the transmutation of substances, seeking to convert base metals into noble ones, like gold which relates to the broader alchemical goals of reaching a higher wisdom and uncovering the secrets of the universe. The notion of the Sahara transforming from a sea into a desert over time can be seen as a metaphor for the alchemical transformation, symbolizing the potential of alchemy to change and evolve the material world.