Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 5

Sanskrit:

अणुर्द्वौ परमाणु स्यात्त्रसरेणुस्त्रय: स्मृत: ।जालार्करश्म्यवगत: खमेवानुपतन्नगात् ॥ ५ ॥

ITRANS:

aṇur dvau paramāṇū syāttrasareṇus trayaḥ smṛtaḥjālārka-raśmy-avagataḥkham evānupatann agāt

Translation:

The division of gross time is calculated as follows: two atoms make one double atom, and three double atoms make one hexatom. This hexatom is visible in the sunshine which enters through the holes of a window screen. One can clearly see that the hexatom goes up towards the sky.

Purport:

The atom is described as an invisible particle, but when six such atoms combine together, they are called a trasareṇu, and this is visible in the sunshine pouring through the holes of a window screen.