187. The final s of the first member

The final s of the first member of a compound often becomes ṣ after an alterant vowel: thus, the s of a prepositional prefix, as niṣṣídhvan, duṣṭára (for duṣṣtára), āvíṣkṛta; and, regularly, a s retained instead of being converted to visarga before a labial or guttural mute (171 a), as haviṣpā́, jyotiṣkṛ́t; tapuṣpā́.