I introduced into a silk bag four quarts of well washed carbonic acid produced from carbonate of ammoniac[212] by heat, and after a compleat voluntary exhaustion of my lungs, attempted to inspire it. It tasted strongly acid in the mouth and fauces, and produced a sense of burning at the top of the uvula. In vain I made powerful voluntary efforts to draw it into the windpipe; at the moment that the epiglottis was raised a little, a painful stimulation was induced, so as to close it spasmodically on the glottis; and thus in repeated trials I was prevented from taking a single particle of carbonic acid into my lungs. [212] Carbonic acid is produced in this way in a high state of purity, and with great readiness. I tried to breathe a mixture of two quarts of common air and three of carbonic acid, without success; it stimulated the epiglottis nearly in the same manner as pure carbonic acid, and was perfectly non-respirable. I found that a mixture of three quarts of carbonic acid with seven of common air was respirable, I breathed it for near a minute. At the time, it produced a slight degree of giddiness, and an inclination to sleep. These effects however, very rapidly disappeared after I had ceased to breathe,[213] and no other affections followed. [213] Carbonic acid possesses no action on arterial blood. Hence perhaps, its slight effects when breathed mingled with large quantities of common air. Its effects are very marked upon venous blood! If it were thrown forcibly into the lungs of animals, the momentary application of it to the pulmonary venous blood would probably destroy life. During the course of experiments on nitrous oxide, I several times breathed Oxygene procured from manganese by heat, for from three to five minutes. In respiring eight or ten quarts; for the first two or three minutes I could perceive no effects. Towards the end, even when I breathed very slowly, my respiration became oppressed, and I felt a sensation analogous to that produced by the want of fresh air; though but little of the oxygene had been consumed. In one experiment when I breathed from and into a bag containing 20 quarts of oxygene for near six minutes; Dr. Kinglake felt my pulse, and found it not altered in velocity, but rather harder than before. I perceived no effects but those of oppression on the chest[214]. [214] In a conversation with Mr. Watt, relating to the powers of gases, that excellent philosopher told me he had for some time entertained a suspicion, that the effects attributed to oxygene produced from manganese by heat, in some measure depended upon nitrous acid suspended in the gas, formed during ignition by the union of some of the oxygene of the manganese with nitrogene likewise condensed in it.