2. It is proved by indubitable evidence that this morbific matter is as capable of entering the system when minute particles of it are diffused in the atmosphere as when it is directly introduced into the blood-vessels by a wound. When diffused in the air, these noxious particles are conveyed into the system through the thin and delicate walls of the air vesicles of the lungs in the act of respiration. The mode in which the air vesicles are formed and disposed is such as to give to the human lungs an almost incredible extent of absorbing surface, while at every point of this surface there is a vascular tube ready to receive any substance imbibed by it and to carry it at once into the current of the circulation. Hence the instantaneousness and the dreadful energy with which certain poisons act upon the system when brought into contact with the pulmonary surface. A single inspiration of the concentrated prussic acid, for example, is capable of killing with the rapidity of a stroke of lightning. So rapidly does this poison affect the system, and so deadly is its nature, that more than one physiologist has lost his life by incautiously inhaling it while using it for the purpose of experiment. If the nose of an animal be slowly passed over a bottle containing this poison, and the animal happen to inspire during the moment of the passage, it drops down dead instantaneously, just as when the poison is applied in the form of a liquid to the tongue or the stomach. On the other hand, the vapour of chlorine possesses the property of arresting the poisonous effects of prussic acid; and hence when an animal is all but dead from the effects of this acid, it is sometimes suddenly restored to life by holding its mouth over the vapour of chlorine. During every moment of life in natural respiration a portion of the air of the atmosphere passes through the air vesicles of the lungs into the blood, while a quantity of carbonic acid gas is given off from the blood, and is transmitted through the walls of these vesicles into the atmosphere. Now that substances mixed with or suspended in atmospheric air may be conveyed with it to the lungs and immediately enter into the circulating mass, any one may satisfy himself merely by passing through a recently painted chamber. The vapour of turpentine diffused through the chamber is transmitted to the lungs with the air which is breathed, and passing into the current of the circulation through the walls of the air vesicles, exhibits its effects in some of the fluid excretions of the body, even more rapidly than if it had been taken into the stomach.