Vagina
the membranous canal leading from the uterus or womb to the extical impossibility of obtaining a per- osopher is obliged to recognize the prac- ternal genitalia. The vagina in lower tical impossibility of obtaining fect vacuum. The so-called Torricellian vacuum, which exists above the mercury column of a barometer, is really filled with mercury vapor probably mixed with a small percentage of air. The very action by means of which the air pump produces its vacuum shows that there can never be an absolute void-a little air always remaining behind. Andrews, following up a suggestion of Davy, obtained remarkable vacua by first pumping in carbonic acid so as to expel as far as possible the air, and then after ordinary exhaustion leaving the carbonic acid to be taken up by moistened caustic potash which has been previously placed in the receiver. In this way Andrews removed all but 350,000 of the gas originally present, whereas by ordinary air pump action only of the gas can be removed. Frankland, Gassiott, Crookes, Dewar, and others, have devised various improvements. Dewar takes advantage of the property of the gases to condense on the surface of solids in a remarkably ingenious way. A piece of carbon, placed in the vacuum vessel, is kept heated while the vessel is being exhausted. After the exhaustion is completed as far as possible, the carbon is allowed to cool so as to permit the small quantity of gas present to condense in its pores. A gentle heating of the carbon at once releases a quantity of the gas, so that this method is invaluable in the study of electric discharges through rarefied gases, since the rarefaction can be so easily altered at will.