 The main difference between these atomisms and Nyāya-Vaiśeṣika atomism lies in their account of combination of atoms. The latter had to resort to God’s will and an unseen force (apparently non-natural) to explain this process of combination. The Jainas and the Buddhists, on the other hand, gave a satisfactory account of the combination of atoms in terms of natural forces. The Jainas, for example, explain the bonding of atoms on the basis of an empirical observation (Tattvārthādhigamasūtra, 5. 32). It is seen that when drops of water fall on particles of barley, one single lump is formed. By generalization, the claim is that a viscid / smooth (snigdha) atom tends to combine with a dry / rough (rukṣa) atom. Viscidity and dryness, smoothness and roughness, are no doubt natural properties of atoms. The following rules of combination are formulated: