Perhaps the most interesting of the whole series are those in which he compared the results obtained _in vacuo_ and atmospheric air. He says: "I put one prepared limb of a frog under the exhausted receiver of an air-pump; it lay on a plate of glass, supported by a cup; zinc was placed beneath the thigh, and gold under the leg; and, by means of a probe passing through a collar of leather, I could touch both metals, so as to excite the muscles to contraction. This I did occasionally, and found the limb capable of excitement for twenty-two hours. The corresponding limb, which was left exposed to the atmosphere, also contracted at the end of that time; so that it was doubtful which of them retained their powers in the greater degree. The same experiment was repeated several times, with results so nearly alike, that I am inclined to believe _irritability_ continues very little longer in common air than it does in the exhausted receiver of an air-pump. "I have frequently produced numerous contractions in the limbs of frogs inclosed in azotic, hydrogenous, and other gases; which likewise tend to show that the cause of irritability does _not depend on oxygen_ for its power of action." He then remarks that, notwithstanding the great importance of oxygen, he thinks it has been overrated; for, says he, "Different tribes of animals partake of it in different degrees; and those who have the least of it are far _from being the least vivacious_." He here reasons on premises which were then universally admitted, and which form at present a portion of many very questionable impressions in relation to respiration. We mention one: "that fish, frogs, &c., breathe less oxygen than warm-blooded animals." But whilst, in respect to the frog, there are many conditions relating to the skin to be considered before we can admit this proposition, we hold it to be _demonstrable_ that fish breathe more oxygen than most other animals; due attention not having been paid to the enormous proportion of oxygen in the air found in water; being in fact, about, one-third. In his concluding remarks, he says, that as regards nitrogen, hydrogen, and carbonic acid, it only shows what we knew before: that they are injurious to life, and that oxygen is not more beneficial than common air. The experiments "showing the long continuance of life and action in muscles in an exhausted receiver, he considers worthy of notice, as tending to show that the cause of irritability in muscles, when once formed, does not require the assistance of external matter."