This liquid is non-toxic liquid paraffin containing a UV sensitive photochromic dye. Indoors it appears as a clear liquid but when exposed to natural light it immediately becomes coloured.
This colour increases in intensity when exposed to direct sunlight. The colour change is reversible, although it takes about 60 seconds for the liquid to become clear again.
Makes a fantastic demonstration of photochromic phenomena.
The same dye chemistry is used in photochromic lenses, special window glass etc. Photochromic cells, for quantitative measurements, can be made, by trapping a drop of the fluid between microscope slides or petri dishes.
The material can be activated by UV LEDs.