 The Buddhists then speculate about the nature of the smallest aggregate. The Sautrāntikas hold that seven atoms form the smallest aggregate. They also maintain that atoms do not touch one another. So the aggregate of atoms is not a solid whole but rather there is space among atoms. Others, however, concede the possibility of dense combination of atoms. The combination of seven atoms takes place in the form of a cluster with one atom at the centre and others around it. The Sarvāstivādins talk about eight types of atom. The four fundamental types are of earth (solid), water (fluid), fire (hot) and air (moving). The secondary atom types are of colour, odour, taste and touch. Thus, according to this view, specific qualities are atomic too. Each secondary atom requires for its support four fundamental atoms. So, by simple calculation, a non-sounding aggregate (aśabda) consists of 20 atoms, while a sounding aggregate (saśabda) is composed of 25 atoms.