Beginning of Essay
The concept of the atom has existed since the 5th century BC, but it was not until the beginning of the 19th century that this concept was developed into a scientific theory. Almost as soon as the modern atomic theory was established, it was discovered that atoms ....

Middle of Essay
.... Bosons, too, may be this same object but in still another state. The everyday world is made of molecules. Because molecules are so small, their existence was not easy to establish. Indirect evidence for molecules was strong during the second half of the 19th century, and the first explicit evidence was found when Brownian motion was correctly understood and studied in about 1906 by physicists Albert Einstein and Jean-Baptiste Perrin. There are many kinds of molecules, and it was clear that simplifying ideas were needed. The Russian chemist Dmitri Mendeleev argued that there was a set of more basic entities of which all molecules are constructe ....