"A few days prior, Priestly had made a curious discovery: He had lit a flame in a large jar, then sealed the container until the flame burned itself out. He then place a mouse in the container and watched as it soon collapsed, apparently due to the lack of air. Had the flame consumed the life-giving gas within the ar? Inspired, Priestly then repeated the same experiment--depleting the air inside the constainer with the candle--but this time, in addition to placing the mouse inside the container, he added a mint plant from the garden, sealed the container quickly before gas could be exchanged with the outside air, and set the container in the sunlight. The mouse regained consciousness. Somehow, the combination of plant and sunlight revitalized the air and infused the mouse with life force. Additionally, he found that the flame would again burn inside the container after the plant had "restored" the air. Life, flame, and air, Priestly realized, were somehow woven together.

For today's experiment he would again use the sun. He magnified the sunlight spilling through the laboratory's only window onto a small amount of a reddish substance known as mercuric oxide. He then used his apparatus to capture the gas that was released as the mercuric oxide began to burn. For the remainder of the day Priestly would perform a series of experiments with the newly isolated gas. He began with the flame. He noticed that it burned with much more intesity when placed inside a container with the new gas. He again filled ethe container with the new gas and sealed the mouse inside. Amazingly, when comparing results to those seen with a container filled with normal air, the mouse stayed conscious four times longer. This new gas, declared Priestly , was "five or six times as good as common air."

"The feeling of it in my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards. 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," he wrote, late in the night.