Probably the best thing about serious historical study of Hong Kong is the impossibility of becoming jaded with the subject. Just when you think you've seen and heard everything, something even more astounding than the previous gem comes along to startle and amuse. Recently, the Antiquities Advisory Board passed a motion to preserve two public lavatories in Central. Although so much local history and heritage has been flushed down the drain over recent decades, a few noteworthy conveniences will now remain. Well-intentioned yet undeniably comic, heritage listings for public toilets are simply the latest in a long line of local preservationist initiatives that, frankly, scrape the bottom of the pan. Let's face it; we've all had at least one 'Oh no! No paper! What now?' moment seared into our consciousness. But do these nasty experiences - and the places where they occur - really constitute an essential aspect of Hong Kong's 'collective memory'? And no apologies are offered for the foregoing gross metaphors; this subject is a columnist's dream come true! Free 'public' conveniences are a relatively recent innovation. In the 19th century, Hong Kong's public lavatories were operated by private contractors, who were awarded the right to gather human excrement in return for an annual payment to the government. Commonly known as night soil, collected excreta was sold for use as agricultural fertiliser. Night-soil collection was the preserve of women known as yeh heung por - 'night fragrance hags'. In the absence of reliable census figures - almost impossible to compile when free movement to and from the mainland was the norm - calculations based on the annual night-soil volume were used to help determine how many people lived in the colony. By dividing the weight collected, and allowing for wastage through water closets, sampans emptying their chamber pots over the side and so on, reasonably accurate population estimates could be reached. The 1921 census was considered quite inaccurate, based on figures obtained from a study titled 'Sanitation in War', which dealt with hygiene measures in the first world war. A rice diet was thought to result in a heavier daily yield than a conventional European diet, therefore fewer people were in Hong Kong than previously thought. An entire section in the 1931 census was devoted to debunking the previous decade's night soil-based population projections. What did this prove? Only that Hong Kong's vital statistics were - much like those of today - ultimately derived from the contents of a crock of you-know-what.