He next made several experiments, by placing the limbs of frogs in nitrogen and hydrogen: the limbs in nitrogen lost their irritability in about two hours and a half; those in hydrogen, in about four hours.
Experiments then follow which consisted in placing other limbs in carbonic acid and nitrous gases respectively; when he found that in both cases the muscles ceased to act in an hour and a half.
He also placed limbs in carburetted hydrogen, and found that they ceased to act after the same period. In other experiments, he found the correctness of Fontana's results; viz. that limbs placed under water, and which had lost their irritability, had for a time recovered it by exposure to air and moisture.
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."
Lastly, he gives an experiment on the blood (which shows how he was working in every direction), in aid of the opinion that the blood derives its scarlet colour from the action of oxygen. "I took the coagulum of venous blood left in a basin after bleeding, and, turning it bottom upwards, waited till its surface had become of a scarlet colour. I then took slices of this surface, and similar slices of the interior part of the coagulum, which had a very dark appearance, and exposed them repeatedly to azotic and nitrous gases. The scarlet colour gradually faded upon such exposure; and the azotic gas being afterwards examined, was found to contain oxygen, while nitrous gas was much diminished, doubtless by combining with the same principle. The gases to which the dark-coloured blood was exposed underwent no change in this experiment. That blood takes oxygen from the air, when it becomes florid, will not, I suppose, be denied, and the experiment I have related shows that it will again part with it, though slowly, without any alteration in its temperature."