Both Uses of

efficacious
in
The Count of Monte Cristo

He has but one fault, he is somewhat wilful; but really, on referring for the moment to what he said, do you truly believe that Mithridates used these precautions, and that these precautions were efficacious?†
Chpt 51-52 *

efficacious = effective in producing an intended effect

"Alas!" exclaimed Mercedes, "if it were so, if I possessed free-will, but without the power to render that will efficacious, it would drive me to despair."†
Chpt 111-112