Verse 19

शब्दस्पर्शस्वरूपाभ्यां संघाताज्जन्यतेऽनलः ।मनस्तद्धनतां प्राप्य ततो भावयति क्षणात्

In plain words

whose very natures are sound and touch, fire is born through their combining. Mind, having attained the density of that, then conceives, in an instant,