opportunity to learn the secret that was hidden within mercury’s powdery
granules.
When a scientist applies a flame to a crucible of mercury calx powder,
an intense fire is generated above the substance as it turns into mercury.
Priestley decided to try and turn the powder into liquid mercury in his
glass bell testing chamber to investigate the “air” that somehow caused
such a brilliant flame upon the embryonic quicksilver. But how to liquify
the powder inside the glass chamber? Cleverly, Priestley used a twelve-
inch “burning lens,” a magnifying glass that concentrated the power of the
sunlight into a small patch of calx, like so many mischievous middle-
schoolers tormenting ants. Holding the lens on the outside of the jar and
melting the powder into liquid mercury, Priestley was able to enrich the air
inside his bell jar, permitting a candle inside the vestibule to burn brighter
or for a mouse to live much longer while entombed inside the airtight trap.
The enriched air byproduct was similar to Joseph Black’s “fixed air,” in
that some type of gas was emitted during a chemical reaction, whether it
occurred during the melting of a powdery solid, or during the exposure of
a stone to acid. Sitting in his small experimental room in the Bowood
estate (preserved to this day), Priestley pondered the significance of the
air. “The feeling of it in my lungs,” Priestley wrote, “was not sensibly
different from that of common air, but I fancied that my breast felt
peculiarly light and easy for some time afterward. Who can tell but that in
time, this pure air may become a fashionable article in luxury. Hitherto
only two mice and myself have had the privilege of breathing it.”^12
Priestley was, of course, breathing super-concentrated oxygen. Mercury
calx is actually mercuric oxide, and when it is heated, 0 2 is given off into
the air.
Joseph Black’s fixed air? It is carbon dioxide (CO 2 ). And why do plants
inside a glass bell jar thrive when exposed to fixed air? Because they are
able, through photosynthesis (also discovered by Joseph Priestley), to
cleave the carbon and give off the oxygen molecule. That, by the way, is
the answer to Jan van Helmont’s willow quiz. The willow tree grew larger,
not by the soil, and not by the water, but by the air around it supplying
CO 2. All of the trees and plants in our world grow larger by converting
CO 2 into the carbonaceous stuff of their frames.