arsenic
磇 centers on the element arsenic, a toxic metalloid historically used in poisons and medicines.
磇 is a phono-semantic compound: 石 (stone, mineral) suggests a mineral substance, while the right side 比 over 口 likely contributes the sound. The character was created specifically for the element arsenic.
The stone radical 石 hints at a mineral, and the right side looks like a comparison 比 of mouths 口—imagine comparing two mineral samples, one of which is the deadly arsenic.
For ヒ, picture a heap of arsenic-laced stones: heap -> ヒ, and the stone radical 石 reminds you it's a mineral.
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