Why does Casca use the word "alchemy" in assessing the value of Brutus to the conspiracy?
At the end of Act I, Scene III, Casca is speaking to Cassius, who has just revealed his plan to convert Brutus to their scheme. Casca is in full agreement, and he comments:
O, he sits high in all the people's hearts,
And that which would appear offense in us,
His countenance, like richest alchemy,
Will change to virtue and to worthiness.
The reference to Alchemy, or the ancient and discredited science of transmutation, touches at least two significant points.
First: Brutus is a beloved member of society, and his involvement in any plan or idea gives it credibility. In this case, Casca is remarking that if the public is initially against their plan, their opinion will turn -- "like richest alchemy" -- when they see Brutus is involved. This interpretation is implicit in the text.
Second: Brutus himself is not fully on board with the scheme. His opinion is actively being swayed by Cassius and others, but he does not know the full extent of the plan, nor has he agreed to be an active part of it. Casca hopes that Brutus, on having the entire plan explained, will change his own opinion -- "like richest alchemy" -- and give his blessing. This interpretation is less implicit, and must be extrapolated from context.
The alchemists were noted for trying to turn lead into gold. Brutus would have the effect of turning the conspiracy into pure gold if he were to join with the other conspirators. His countenance will not change but will change the appearance of the conspiracy in the eyes of the Roman citizens. His countenance (or involvement) will change "that which would appear offense in us" (the actual assassination of Caesar).