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is as mucli oxygen present as there is in atmospheric air; secondly, one bron-chial tube of the land-tortoise may be tied, without any serious injury to theanimal; but if, instead of tying it, the animal be made to inhale carbonic acid gasby it, death takes place in a few hours; (Christison, Treatise on Poisons, p. 745,3d. edit.) and, thirdly, " the cases of insidious poisoning by small doses of car-bonic acid scarcely admil of explanation, save on the grounds of the essentiallyand specifically poisonous action of carbonic acid gas, when sufficiently dilutedto become respirable." 1 The impression produced on the pulmonary extremitiesof the par vagum, by the carbonic acid in the lungs, is supposed by some phy-siologists to be the ordinary Stimulus to inspiration. (Dr. M. Hai] on the Dis-eases and Derangements of the Nervous System, p. 66. Lond. 1841.) Ac-cording to the experiments of Nysten, (Retlierches, p. 88.) this gas may beinjected into the nervous System in large quantity, without stopping the circula-tion, and without acting primitively on the brain; but when more is injected thanthe blood can dissolve, it produces death by distending the heart, as when air isinjected into the veins (see p. 23.) Applied to the skin of animals, free accessof common air to the lungs being preserved, it produces, if the experiment becontinued long enough, death.

y. On Man. —If an attempt be made to inhale pure carbonic acid gas, theglottis spasmodically closes, so as to prevent the smallest porlions from enteringthe lungs. (Davy, Ilcsearches, p. 472.) When mixed with more than twiceits volume of air, this gas ceases to provoke spasm of the glottis, and may betaken into the lungs. It then acts as a narcotic poison. Its specific influenae isexercised on the central organs of the cerebro-spinal System, which it probablygains access to, through the medium of the blood. Its action on the nervousSystem does not depend on its impeding the arterialization of the blood; becausedeath may occur from the respiration of an atmosphere containing suflicient oxy-gen to support life per se, but with which is mixed carbonic acid gas. It is im-possible to State the maximum quantity of this gas which may be present in theair without exciting its effects: it probably varies for different individuals, somepersons being mucli more susceptible of its action than others. If the proportionof carbonic acid be large, the effects are almost immediately developed; whereas,if the proportion be small, they are very slowly manifested. Dr. G. Bird hasshown that an atmosphere, containing five per cent. of carbonic acid, provedfatal to a bird in thirty minutes; and it is probable that the continued respirationof an atmosphere containing a very considerably smaller proportion of carbonicacid, would be attended with dangerous and even fatal consequences.

The earliest symptom usually experienced by persons exposed to an atmo-sphere containing carbonic acid gas is throbbing headach, with a feeling of ful-ness and of tightness across the temples, and in the occipital region. Giddiness,loss of muscular power, a Sensation of tightness at the ehest, augmented actionof the heart, and often palpitation, sueeeed. The ideas become confused, and thememory partially fails. A buzzing noise in the ears is next experienced; visionis impaired; and a strong tendency to sleep sueeeeds, or actual syncope ensues.The pulse falls below its natural Standard, the respiration becomes slow andlaborious, the surface cold and often livid, but the eyes retain their lustre. Con-vulsions, sometimes aecompanied with delirium, foaming at the mouth, andvomiting, come on, and are terminated by death. On post-mortem examination,engorgement of the cerebral vessels, and sometimes serous or even sanguineouseffusion, are the usual appearances. (For farther details, I must refer the readerto Dr. Bird's paper before cited.)

Applied to the skin (care being taken that it be not inhaled) it produces a Sen-sation of warmth and prickling or tingling, sometimes aecompanied by pain, in-creased frequency of the pulse, sweating, and excitement of the nervous System.

• Dr. Gohiini; Bird's experiments, in the Guy's Hospital Reports, vol. iv. p. 75; also London MeiicalGazct'sN. S. vols. i. and ii. for 1838-0.

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