ṣ is substituted for the s of 1. stuta 2. stoma and 3. soma when preceded by agni in a compound.,
Thus अग्निष्टुत्, अग्निष्टोमः, अग्नीषोमः ॥
Ishti:- The इ of अग्नि is lengthened before सोम and it is after such lengthened ई, that the स of सोम is changed to ष, otherwise not. As अग्निसोमौ माणवकौ ॥ So also अग्निसोमौ तिष्ठतः (where 'agni' means fire, and 'soma' a kind of herb) \the fire and the soma plant are here.\
When there is no compounding we have अग्ने सोमः ॥
The word अग्निष्टुत् is formed by क्विप्, the sacrifice in which Agni is praised (स्तूयते) is so called. अग्निष्टोमः is also the name of a sacrifice: the first division (संस्था) of the Soma-yaga. अग्नीषोमः is a देवता-द्वन्द्वः so where there is no Devata-Dvandva, the ष change will not take place. Thus where Agni and Soma are names of two boys, or where they refer to physical fire and herb. According to Asvalayana there is lengthening and ष change in the last case also, as अग्नीषोमौ प्रणेष्यामि ॥,