4SO THE SCIENTIFIC MONTHLY
if not absolutely pure air, such air as could sustain life for some minutes, long enough to enter a burning house or, as he suggested, a laboratory filled with noxious fumes. The mechanism was a bladder divided up into compartments by four diaphragms of flannel or linen soaked in a solution of potash or '^sal tartar'^ capable, as we now know, of absorbing the respiratory carbon dioxide. A tube led from the far end of the bladder and, curving upwards, terminated in a mouthpiece near which were placed two valves, one allowing air to enter the near end of the bladder, the other preventing it from passing back into the far end. The nostrils had to be closed, as they have to be in all mouth-breathing forms of such apparatus. The receiver held between four and five ^'quarts of air^'; Hales thought that with one gallon of air and four diaphragms, respiration could be supported for at least five minutes. He remarks, what we can readily believe, that there was much discomfort unless the valves worked easily.^
In view of the prominence which life-saving apparatus has attained at the present day, it seems exceedingly interesting to know that be- fore 1726, a practical attempt had been made to construct an artificial rescue-apparatus. Hales himself contemplated its use not only in the foul air of mines, but by divers under water ; it is, however, very doubt- ful whether the mechanism as he left it could have been used under water. This simple invention is the humble parent of the various in- genious life-saving apparatuses of the present day, the Fleuss, the Drager, and others which enable a man to remain for upwards of two hours in atmospheres not merely poisonous, but actually deadly. As we have already seen, it was this invention of Hales that inspired Black to discover respiratory carbon dioxide.
As all men of science know, Stephen Hales may be said to have founded the science of experimental, botanical physiology ; his observa- tions by means of mercury manometers on the pressure exerted by the rising of sap in vines are classical. Although he was a pioneer, he fully acknowledged the work done in plant physiology by Mayow, Grew and Malpighi. His views regarding the transpiration of plants and their nourishment, and how they utilized some constituent of the atmos- phere only under the influence of solar light, were all in advance of his time.
In animal physiology he was also a pioneer, for he was the first to ascertain the magnitude of the pressure of the circulating blood: this he did by opening the left crural artery of a living animal, the horse. His method was crude : he merely allowed the blood to rise as high as it could (8 ft. 3 in.) in a vertical tube partly of brass and partly of glass; but the principle of his method is even now most fruitfully used in practical medicine. He studied the general physiology of the blood- pressure in arteries and in veins, as also the force and output of the
iCf. "Vegetable Statics, Vol. I., p. 265.
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