Highly porous silicon reacts with water vapor in the air to create ima
ges on photographic plates within seconds. It is demonstrated that thi
s process, known as chemography, can be used diagnostically to reveal
inhomogeneity in porous silicon layers that is not always detectable b
y optical or electron microscopy. It is suggested that many dried high
ly porous semiconductors may constitute a significant health hazard ow
ing to the liberation of high concentrations of toxic hydrides through
hydrolysis.