Colloids are no chemical class of substances; we are dealing rather with a certain state of matter, like the condensed states of the solid or liquid. What is so special about this state is that macroscopic properties are greatly affected by size in these dimensions.
For more than 150 years, chemists have been exploring the colloidal state of matter, characterised by Ostwald as a “world of neglected dimensions”. Selmi described aqueous dispersions of silver chloride, sulfur, and Prussian blue in 1845. Soon afterwards, Faraday examined gold sols and deduced that this (colloidal) state of matter must be thermodynamically instable and that stabilisation must be a kinetic phenomena. Some of the dispersions prepared by Faraday are still on display in the British Museum today.
In the beginning of the twentieth century, colloidal science established a connection between (preparative) chemistry and (theoretical) physics. Einstein discovered the connection between Brownian motion and the diffusion coefficient while Perrin used this relationship to calculate Avogadro’s number. Ever since, colloidal science has linked various disciplines of science, such as biology and the materials sciences, to name but two.