In the cold winter of 1835, I took, at Troy, the old summer stage, at midnight, to cross the Green Mountains. I was alone in the large and ill-closed vehicle; the thermometer was sinking as I proceeded on my way, until it had reached 25° below zero, a degree of cold to which I had never before been subjected. When I had traveled alone twenty miles, I found myself in imminent danger of perishing. Ordinary expedients to get warmth were no longer availing; numbness and cold at the vitals were overcoming me; and I knew that to give way to them was to die. I thought of my theory; but I was fearful that I should commit sin if I tampered with the sacred "breath of life." But my necessity was urgent, and I aroused, stood up, and breathed that dense air with violence. It felt for the moment cold to my lungs, but soon came heat with a rush, and with it pain, as if the whole surface of the throat and lungs were blistered; and my first thought was that I should die, justly punished for my temerity. But soon I was restored to genial warmth; and rejoiced in having successfully made an important physiological experiment. Afterwards, having been instrumental in relieving a woman who was perishing from having breathed the fumes of charcoal, I was led to reflect that in such cases there was something to be taken away from the lungs, as well as warmth to be added. This woman's extreme coldness, and feeble, fluttering pulse, showed that she was dying for want of right breathing; and in her case there was no doubt that the cause was the same as that of death by drowning. The carbonic acid gas which she had inspired, being heavier than atmospheric air, settled as water in her lungs, and in the same manner prevented the access of oxygen to their living tissues. And hence arose the reflection that the ordinary carbonic acid gas, which is always the residuum of respiration, might, from weakness, settle in the lungs, and thus become the cause of disease and death. The presence of carbonic acid in the lower bronchial tubes and cells, existing in quantities sufficient to prevent the natural combustion by breathing, was brought to my mind in March, 1847, while searching for the cause of an agonizing paroxysm of sick headache. The distressed feelings of obstructed life with which I was tossing and struggling, together with the agonizing pain in the head and pressure on the stomach, might well arise from such a cause. Standing (for position is important) in a full current of air from an open window, I commenced a species of violent artificial breathing, for the purpose of ejecting the supposed heavy gas, and filling my lungs with pure air. This was done by contracting the chest on every side to its smallest possible dimensions, and at the same time throwing out the air violently and from the bottom of the thorax, as if under the operation of an emetic; then alternating by opening the chest to its greatest capacity, and drawing