Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 36

Sanskrit:

मृदुत्वं कठिनत्वं च शैत्यमुष्णत्वमेव च ।एतत्स्पर्शस्य स्पर्शत्वं तन्मात्रत्वं नभस्वत: ॥ ३६ ॥

ITRANS:

mṛdutvaṁ kaṭhinatvaṁ caśaityam uṣṇatvam eva caetat sparśasya sparśatvaṁtan-mātratvaṁ nabhasvataḥ

Translation:

Softness and hardness and cold and heat are the distinguishing attributes of touch, which is characterized as the subtle form of air.

Purport:

Tangibility is the proof of form. In actuality, objects are perceived in two different ways. They are either soft or hard, cold or hot, etc. This tangible action of the tactile sense is the result of the evolution of air, which is produced from the sky.