Two liquids, one boil
Pure water boils at 100 °C at sea level, pure ethanol at 78.4 °C. A mixture does not boil twice, first at 78 and then at 100: it boils once, somewhere between, and the lower curve on the chart shows where. A wash of 8% boils at about 93 °C; the 25% low wines in a spirit still at about 87 °C.
At that temperature both kinds of molecule are escaping into the vapour, but not equally. Ethanol molecules hold on to their neighbours less tightly than water molecules do, so they leave more readily. The vapour over a liquid of 10% alcohol is about half alcohol: read it off the upper, red curve, straight across from the boiling point.
Condense it and you have spirit
Cool that vapour and it becomes a liquid of the vapour's strength. This is the whole of distilling: one boil and one condensation give one step of enrichment. A malt whisky distillery takes two such passes. The wash still turns an 8% wash into low wines of roughly 20–25% (the run starts much stronger, but weakens as the pot loses its alcohol), and the spirit still turns those into new make of around 70%.
A note on the numbersThe chart is drawn from the standard table for alcohol and water at one atmosphere, measured in mole fractions and converted here into approximate % by volume. Alcohol and water shrink a little when mixed, so the strengths are good to a percent or two. In the glass cell each dot is a molecule: alcohol molecules are bigger than water molecules, so a liquid of 10% by volume holds only about 3 alcohol molecules in every 100.
The road narrows
Press Condense, boil again to climb the staircase. Each step gains less than the one before, because the two curves close in on each other as the mixture gets stronger. They finally touch near 97% alcohol by volume (95.6% by weight), where the liquid boils at about 78.2 °C, a whisker below pure ethanol. This is the azeotrope: the vapour there has exactly the strength of the liquid, so distilling can no longer separate them. Every neutral spirit on earth stops at or just short of this point; absolute alcohol for laboratories is dried by other means.
What the thermometer says
A thermometer in the still head reads the temperature of the vapour arriving there, which is set by its make-up. That is why the stillman can follow a run by temperature: as the pot runs out of alcohol, the vapour gets weaker and its temperature climbs from the high seventies towards 100 °C. In the still house on the front page the same rise shows on the head thermometer. Below, in chapter 4, a column turns this staircase into a tower.