S. S. Van Dine (1888–1939)

“Markham, old dear, how do those robust lads ever succeed in running down a culprit?”
“You have witnessed only the barest preliminaries,” Markham explained. “There are certain things that must be done as a matter of routine— ex abundantia cautelæ*, as we lawyers say.”
“But, my word!—such technique!” sighed Vance. “Ah, well, quantum est in rebus inane**! as we laymen say.”

The girl, in a simple white skating costume, seemed unreal in the light of the moon and stars. She was going through one difficult skating figure after another with serious repetition, as if trying to perfect their intricacies. Vance suddenly became attentive.
“My word!” he whispered. “Magnificent skating!”
He stood fascinated by the girl’s proficiency as she executed various school figures and complicated free routines.
The phonograph ran down and, as the girl completed an involved jump and spiral spin, Vance approached her with a cheerful greeting. At first she was startled; then she smiled shyly.

Then Markham turned back to Vance. “And as for you,” he said with good-natured effrontery, “I think you’re a raving maniac.”
“Granted,” said Vance. “No de lunatic inquirendo* writ necess’ry.”
…
“What a beastly matutinal odyssey, Sergeant!” Vance shuddered dolefully. “And what befell when you came at last to the hut of Eumœus**?”
“The guy’s name is Robbin, like I told you. And he don’t live in a hut…”
…
“I don’t blame you, Mr. Vance. I’m hot and annoyed. Maybe I do sound as if I was messing around with ancient Egyptians, and mandragora, and viper venoms, and secret Gypsy potions, and witches’ ointments with their henbane, and Borgia poisons, and Perugia water, and Aqua Tofana.”
…
“The Tofana the doctor mentioned died in Sicily in the seventeenth century. And she wasn’t a fortune-teller. Far from it. She devoted her talents to mixing a liquid which has since come to be known by her name. Aqua Tofana was a deadly poison; and this woman plied her poisoning trade on such a wholesale scale that the name of her concoction has never been forgot. Though her mixture was probably nothing but a strong solution of arsenic, there’s still a lot of mystery attaching to it.”

To be sure, the motive for the crime, or, I should say, crimes, was the sordid one of monetary gain; and superficially the technique was similar to that of the numerous cases in the same category. But through Vance’s determination and fearlessness, through his keen insight into human nature, and his amazing flair for the ramifications of human psychology, he was able to penetrate beyond the seemingly conclusive manifestations of the case.