The instability of the compound of hæmoglobin and oxygen is shown under the air-pump. The pressure of air in the open equals 760 millimetres of mercury. When the pressure falls to about 250 millimetres, the oxygen is rapidly given off. This is a matter of considerable interest in its bearing upon the question of the height to which it is possible for a human being to ascend. An animal placed in a chamber from which the air is pumped dies when the pressure falls to 250 millimetres of mercury. It has been ascertained that a man under the same circumstances can bear with impunity a reduction to 300 millimetres. How much lower must the pressure fall before it proves fatal? Of three aeronauts who ascended in the balloon _Zenith_ to a height of 8,600 metres (26,500 feet), two died. The third, Tissandier, became unconscious, but recovered during the descent. The pressure of the atmosphere at such a height is 260 millimetres. The greatest mountain heights yet attained are 23,100 feet (Aconcagua, in the Southern Andes), reached by Fitzgerald, and 23,400 feet (Trisul, in the Garhwal Himalayas), reached by Dr. Longstaff and his companions. The pressure at this height was 320 millimetres. From these facts it is clear that mountaineers have just about reached the limit; but since they have not as yet mounted to a height at which the barometric pressure is less than 300 millimetres, it is possible that slightly higher mountains are still waiting to be conquered. At 23,000 feet the oxygen contained in arterial blood does not exceed 10 volumes per cent. (_cf._ p. 190). It is therefore about half the normal amount. Hence the breathlessness and sense of feebleness experienced by climbers. The least exertion leads to the consumption of all the circulating oxygen. But since the effects of want of oxygen are felt at altitudes much lower than those to which reference has been made, it is clear that the question cannot be regarded as simply one of physics. The nervous system suffers when an attempt is made to do work with a deficient oxygen-supply. Violent headache and nausea attack most persons long before a level is reached at which the combination of hæmoglobin with oxygen ceases to be possible. The occurrence of this “mountain sickness” reminds us that we must not take for granted that the nervous system will continue to do its work right up to the altitude at which oxyhæmoglobin is dissociated. Still, the figures show that, apart from these nervous symptoms, which disappear after a time, no serious disturbance occurs even though the atmospheric pressure be but little higher than the absolute minimum at which hæmoglobin combines with oxygen. The capacity of the blood for rapidly absorbing oxygen in the lungs and readily parting with it to the tissues is easily and completely explained by the property which hæmoglobin possesses of forming an unstable compound with this gas.