Urea: Separated from Urine as the Oxalate, Freed with Chalk and Crystallised

Urine holds 1.5–2 % urea. Thorpe's Dictionary (1913) gives the usual way to get it out: the concentrated, filtered urine is treated with hot saturated oxalic acid, which throws down the sparingly soluble urea oxalate. Chalk breaks the oxalate up again; the liquid is cleared with animal charcoal and evaporated until urea crystallises on cooling, in needles or four-sided prisms. The picture: our diagram.