Factual error: Using a Draeger tube to test for nitric acid fume, the instrument made a clicking sound. This device does not click. The bellows pump is squeezed drawing air through the tube. A reaction takes place between the air contaminant and the material in the tube causing a color change. The length of stain is proportional to the concentration.
Factual error: Nitric acid is a liquid and not a gas. The gaseous precurser is NO2 which combines with water forming nitric acid, HNO3. Neither causes instant death.
Answer: Earlier in the episode, Horatio tells Mac that Penrod was seen in Tribeca getting his leg set, only to bolt when he learned a nurse wised up the cops. Caine and the NYPD probably got Penrod's address from the hospital.
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