Vaisesika Sūtra 7.1.04

तेजसु तेजसि वायौ च नित्या द्रव्यनित्यत्वात् ॥४॥

tejasu tejasi vāyau ca nityā dravya-nityatvāt ॥4॥

[तेजसु (tejasu) - in light; तेजसि (tejasi) - in fire; वायौ (vāyau) - in air; च (ca) - and; नित्याः (nityāḥ) - eternal; द्रव्य (dravya) - substance; नित्यत्वात् (nityatvāt) - due to eternality;]

The light in fire, and air, are eternal due to the eternality of the substance.