PRESCRIP′TIONS. Recipes or formulæ for the preparation and exhibition of medicines intended, generally, for immediate use. See Prescribing (above).
PRESERVES′. A general term, under which are included the various fruits and vegetables which are seasoned and kept in sugar or syrup, more especially those which are so preserved whole or in slices. See Candying, Jam, Marmalade, &c.
PRESS (Correcting for the). See Proofs.
PRESSURE, BAROMETRIC, on the Phenomena of Life. M. Bert has contributed to the ‘Comptes Rendus’[122] (lxxiii, 213, 503; lxxiv, 617; lxxv, 29, 88) an account of the following experimental researches on the influence of changes in the Barometric Pressure on the Phenomena of Life:—
[122] ‘Journal Chemical Society,’ vol. xxv.
He finds that at pressures under 18 centimètres of mercury animals die from want of oxygen; at a pressure of one to two atmospheres, from want of oxygen and presence of carbonic acid; at 2-6 atmospheres, from the presence of carbonic acid alone; at 6-15 atmospheres, from the presence of carbonic acid and of excess of oxygen; and at 15-25 atmospheres, from the poisonous action of oxygen alone.
Animals die from want of oxygen when the amount contained in their arterial blood is not sufficient to balance a pressure of 3·5 per cent. of oxygen in the atmosphere. They die from poisoning by carbonic anhydride when the amount contained in their venous blood is sufficient to balance a pressure of 26 to 28 per cent. of carbonic anhydride in the atmosphere in the case of sparrows, of 28 to 30 for mammals, and of 15 or 16 for reptiles.
As the pressure of oxygen in the surrounding air depends on two factors, the percentage proportion and the barometric pressure, the barometric pressure may be reduced to 6 centimètres for sparrows, if the proportion of oxygen in the air is increased; and it may be raised to 23 atmospheres without causing death, if the proportion of oxygen is reduced by mixing the air with nitrogen. Aeronauts might, therefore, ascend higher than it has hitherto been possible to do by taking with them a bag of oxygen to inhale; and the danger that threatens divers of being poisoned by the oxygen in the compressed air might be averted by using a mixture of air and nitrogen.
From an examination of the gases in the blood of animals confined in rarefied air the author finds that both the oxygen and the carbonic anhydride in the blood diminish. The dyspnœa which is felt in ascending mountains
is therefore due to want of oxygen in the blood. The diminution in oxygen becomes diminished at 20 centimètres pressure, yet this is the pressure under which the inhabitants of the elevated Mexican plateau of Anahuac live. The oxygen diminishes more quickly and more regularly than the carbonic anhydride. Although there are but very small quantities of gases simply dissolved in the blood, the chemical combinations in which they take part are dissociated very easily and in a progressive manner under the influence of diminished pressure, and this dissociation takes place more easily in the organisms than in experiments in vacuo.