For millennia, most cities had been bound inexorably to the natural ecosystem that lay outside their walls: the energy flowing through the fields and forests around them established a population ceiling they couldn’t grow beyond. London in 1854 had shot through those ceilings, because the land itself was being farmed more efficiently, because new forms of energy had been discovered, and because shipping and railway networks had gready expanded the distance that energy could travel. The Londoner enjoying a cup of tea with sugar in 1854 was drawing upon a vast global energy network with each sip: the human labor of the sugarcane plantations in the West Indies and the newly formed tea plantadons in India; the solar energy in those tropical realms that allowed those plants to flourish; the oceanic energy of the trade currents, and the steam power of the railway engine; the fossil fuels powering the looms in Lancashire, making fabrics that helped fund the entire trade system.
The dramatic increase in people available to populate the new urban spaces of the Industrial Age may have had one other cause: tea. The population growth during the first half of the eighteenth century neatly coincided with the mass adoption of tea as the de facto national beverage. (Imports grew from six tons at the beginning the century to eleven thousand at the end.) A luxury good at the start of the century, tea had become a staple even of working-class diets by the 1850s. One mechanic who provided an account of his) weekly budget to the Penny Newsman spent almost fifteen percent his earnings on tea and sugar. He may have been indulging in it fc the taste and the salutary cognitive effects of caffeine, but it was a healthy lifestyle choice, given the alternatives. Brewed tea posses several crucial antibacterial properties that help ward off waterborne diseases: the tannic acid released in the steeping process kills off those bacteria that haven’t already perished during the boiling of the water. The explosion of tea drinking in the late 1700s was, from the bacteria’s point of view, a microbial holocaust. Physicians observed a dramatic drop in dysentery and child mortality during the period. (The antiseptic agents in tea could be passed on to infants through breast milk.) Largely freed from waterborne disease agents, the teadrinking population began to swell in number, ultimately supplying a larger labor pool to the emerging factory towns, and to the great sprawling monster of London itself.
Steven Johnson's books are always orientated towards network thinking. This passage is dealing with the surrounding circumstance of the 1854 cholera outbreak in London that lead to John Snow creating a map of deaths. This helped him trace the source of disease to a particular water pump, proving that cholera is waterborne when it was previously speculated as airborne. Key point to take away here - a map of cholera deaths is not just a map - it is a heterogeneous assemblage of West Indian sugar, Indian Tea (and the shipping technology allowing them to get to England), heated water, exploding populations and so on. The difficultly with emergence is how it generates 'a sum of its parts' way of understanding things, which tends to cause hierarchy where a flat approach deals with things in a more useful way - performances of things aligning to generate entities.