“With the greatest esteem, I am Sir, yours sincerely “J. PRIESTLEY.”
In Davy’s next contribution, “On the Silex composing the Epidermis, or External Bark, and contained in other parts of certain Vegetables,” published in Nicholson’s Journal in the early part of 1800, we find the evidence of a chastened and contrite spirit. The theme is humble enough, and the language as sober and sedate as that of Mr. Cavendish. The chance observation of a child that two bonnet-canes rubbed together in the dark produced a luminous appearance, led him to investigate the cause, which he found to reside in the crystallised silica present in the epidermis. Reeds and grasses, and the straw of cereals, were also found to be rich in silica, from which he concludes that “the flint entering into the composition of these hollow vegetables may be considered as analogous to the bones of animals; it gives to them stability and form, and by being situated in the epidermis more effectively preserves their vessels from external injury.” It is doubtful, however, whether the rigidity of the stems of cereals is wholly due to the silica they contain.
From a letter to Mr. Davies Gilbert, dated April 10th, 1799, we learn that he had now begun to investigate the effects of gases in respiration. In the early part of the year he had removed to a house in Dowry Square, Clifton, where he had fitted up a laboratory. After thanking his friend for his critical remarks on his recently published essays, he says:
“Your excellent and truly philosophic observations will induce me to pay greater attention to all my positions.... I made a discovery yesterday which proves how necessary it is to repeat experiments. The gaseous oxide of azote is perfectly respirable when pure. It is never deleterious but when it contains nitrous gas. I have found a mode of obtaining it pure, and I breathed to-day, in the presence of Dr. Beddoes and some others, sixteen quarts of it for near seven minutes. It appears to support life longer than even oxygen gas, and absolutely intoxicated me. Pure oxygen gas produced no alteration in my pulse, nor any other material effect; whereas this gas raised my pulse upwards of twenty strokes, made me dance about the laboratory as a madman, and has kept my spirits in a glow ever since. Is not this a proof of the truth of my theory of respiration? for this gas contains more light in proportion to its oxygen than any other, and I hope will prove a most valuable medicine.