‘Atomic weight, triads, and Prout’ covers the shift in chemical thinking from qualitative analysis to quantitative analysis. John Dalton was the first to put forward the ideas of modern atomic theory. Avogadro realized that gases could exist as diatomic molecules, and Von Humboldt and Gay–Lussac postulated that these molecules reacted in a ratio. Prout also realised that atomic weight rose in multiples of the weight of hydrogen. Dobereiner introduced the concept of triads of similarly reacting elements, but developments of this system were flawed. It was Kremers who first started to relate elements that were chemically dissimilar, thus paving the way for periodicity.