Why is it symbolic that Jekyll's first supply of the salt was impure?

The fact that the salt was impure allowed for the success of the experiment. As a result..... the conclusion of Jekyll's experiment is an accident..... soemthing that cannot be repeated when the variable is changed.

My provision of the salt, which had never been renewed since the date of the first experiment, began to run low. I sent out for a fresh supply, and mixed the draught; the ebullition followed, and the first change of colour, not the second; I drank it and it was without efficiency. You will learn from Poole how I have had London ransacked; it was in vain; and I am now persuaded that my first supply was impure, and that it was that unknown impurity which lent efficacy to the draught.