it is solid atoms
One of these philosophers was Democritus (c. 460–370 BCE), often referred to as the “laughing philosopher” because of his emphasis on cheerfulness. He taught that there were substances called atoms and that these atoms made up all material things. The atoms were unchangeable, indestructible, and always existed.
One of the first atomic theorists was Democritus, a Greek philosopher who lived in the fifth century BC. Democritus knew that if a stone was divided in half, the two halves would have essentially the same properties as the whole. ... Democritus, theorized that atoms were specific to the material which they composed.
Greek philosophers Leucippus and Democritus first developed the concept of the atom in the 5th century B.C.E. However, since Aristotle and other prominent thinkers of the time strongly opposed their idea of the atom, their theory was overlooked and essentially buried until the 16th and 17th centuries.
so it is solid atoms