SB 3.26.36

मृदुत्वं कठिनत्वं च शैत्यमुष्णत्वमेव च ।
एतत्स्पर्शस्य स्पर्शत्वं तन्मात्रत्वं नभस्वतः ॥३६॥

mṛdutvaṁ kaṭhinatvaṁ ca
śaityam uṣṇatvam eva ca
etat sparśasya sparśatvaṁ
tan-mātratvaṁ nabhasvataḥ

mṛdutvam—softness; kaṭhinatvam—hardness; ca—and; śaityam—cold; uṣṇatvam—heat; eva—also; ca—and; etat—this; sparśasya—of the subtle element touch; sparśatvam—the distinguishing attributes; tat-mātratvam—the subtle form; nabhasvataḥ—of air.

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

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.