Sound, touch and form are the three properties of fire; sound and touch are the two properties of the air; and sound is the property of the sky.
शब्दः स्पर्शश्च रूपं च तेजसोऽथ गुणास्त्रयः। शब्दः स्पर्शश्च वायौ तु शब्दश्चाकाश एव तु॥