3. Though enough has already been said to evince that the superior redness of the arterial blood is derived from oxygen gas, we shall still quote two instances from Mr. Hunter’s Treatise on the Blood, which set this forth in the clearest manner; and these instances are the more remarkable, because they demonstrate the phenomena not of the _dead_, but of the _living_ body. 1. A gentleman in an apoplexy, who seemed to breathe with great difficulty, was bled in the temporal artery. The blood flowed very slowly, and for a long time. It was as dark as venous blood. He was relieved by the operation; but, on opening the same orifice in two hours, the blood flowed of the usual florid colour. 2. A lady in an apoplexy was treated in the same manner, and Mr. Hunter observed, that when she breathed freely, the blood from the temporal artery assumed a bright red colour; but when her breathing was become difficult, or when she seemed scarce to breathe at all, it resumed its dark colour, and this several times during the operation. 4. Respiration is not, as Dr. Girtanner says, a process similar to the combustion and oxydation (the calcination) of metals. Some of these by calcination, and _all_ of them in the opinion of Dr. Girtanner, unite with the condensable part of the oxygen contained in the air, while the elastic part is dissipated in flame or heat. The reverse of this takes place in breathing; for here the elastic part of the oxygen unites with the blood, and makes it warm, while the condensable part, uniting with certain particles to be thrown off from the body, passes away in fixed air. Thus the process of respiraton does not resemble the calcination of a metal (at least according to our author’s opinion of that operation) but rather the inflammation of some combustible substance; for in both cases a certain quantity of carbon is found to be united with the basis of oxygen in the atmosphere, and thrown off from the place of combustion; and thus a quantity of fixed air is produced from every burning substance. Just so is it with respiration. If the condensable part of the oxygen combined with the blood, then no fixed air could be produced; or if any part of the oxygenous base was absorbed, it must certainly be known by a proportional deficiency in the quantity of fixed air produced. But there are no experiments made with accuracy sufficient to determine this point. It is true that many very able physiologists, as Borelli, Jurin, &c. have been of opinion, that part of the air is absorbed in respiration; but when we come to particulars nothing can be determined. Dr. Hales by experiment found the quantity absorbed to be a _sixty-eighth_ part of the whole quantity inspired; but, on account of supposed errors, he states it only at _an hundred and thirty-sixth_ part. Between these two the difference is so enormous, that we know not how to draw any conclusion from them. The French chemists are more decisive, and agree pretty well with one another.