By the start of the twentieth century a simple basic understanding of how cloud droplets form had been reached. It is useful to review these findings before going forward to fill in the details learnt during the last century. Recall, for example, the experimental work of Guericke in the seventeenth century with his air pump and cloud chamber in which a few large droplets might form one day, a large number of smaller drops on another. He wrongly concluded that two different processes were in action. A century later Kratzenstein repeated Guericke's experiments but was confused by the colours he saw and wrongly concluded that the drops were bubbles (or ‘vesicles’). In the nineteenth century, Waller collected cloud droplets on spider webs and concluded that they were drops, not bubbles. Then in 1875 the Frenchman, Coulier, while doing experiments with a cloud chamber, deduced correctly that droplets only form if there is ‘dust’ in the air. This finding was ignored at the time, but soon afterwards John Aitken, in Scotland, made a similar discovery. Aitken acknowledged Coulier's work when his attention was drawn to it, but went on to conclude that the fewer the ‘dust’ particles, the larger the drop sizes and the smaller their number, suggesting that the available water vapour was shared amongst the particles. He also concluded that most probably dried sea spray in the form of fine powder was the main source of nuclei, other possible causes being volcanic and meteoric dust, along with smoke from combustion.