THE CAPILLARY POWER OVERCOMES POWERFUL MECHANICAL PRESSURE. 45 



is taking place with very great force, provided the rate of the bodies on both sides of 

 the barrier is the same. It gives erroneous results in all those cases where the me- 

 chanical leakage exceeds the true percolation, and hence has a very limited application 

 in all these experiments. Other means are therefore required to test the passage of 

 fluids, and for this purpose there is no arrangement more convenient than that represent- 

 ed in fig. 18. It consists of a tube, three eighths of an inch in diameter, and several feet 

 long, bent at the point b upward, and expanded at c c into a funnel termination. When 

 the instrument is in action, the longer limb, d b, is filled to some determinate height with 

 mercury, which also rises to a certain distance in the shorter leg, above this, and to the 

 height a d some fluid is placed acting as chemical test of the presence of the gas, in- 

 tended to be passed through the barrier c c, which is tied air tight over the funnel 

 mouth. The following experiment will indicate its use : Having placed this siphon 

 on the mercurial trough, a quantity of mercury was poured into it, sufficient to cut ofF 

 communication between the two limbs; then in the shorter limb a column of litmus 

 water reddened by muriatic acid, and occupying a depth of one eighth of an inch, was 

 introduced; over the funnel mouth a thin lamina of India-rubber was tied, and upon that 

 a piece of stout silk, for the purpose of strengthening the barrier. A column of mercury, 

 forty-three inches in height, was next placed in the long limb, and a jar of ammoniacal 

 gas over the short one. In the course of one minute, a cloud of dark-blue particles was 

 seen descending through the litmus, and in six minutes it had become uniformly blue ; 

 thus proving the passage of ammonia through a tissue of India-rubber, against a pressure 

 of almost one atmosphere and a half. 



147. There were considerable difficulties encountered in the outset of these experi- 

 ments in tying on the India-rubber barriers, so as to withstand the high pressures to 

 which they were exposed without leakage ; an insidious leakage which took place be- 

 tween the sides of the glass and that part of the India-rubber compressed by the string 

 against it. This, however, was effectually prevented by setting fire to a piece of India- 

 rubber, and daubing the semi-fluid material on that part of the glass around which the 

 string was to pass ; then, on tightly binding on the barrier, it came into perfect contact 

 with the glass, and was retained there by the sticky material, no leakage whatever taking 

 place, unless some part of the arrangement burst. 



148. The experiment just related leads to some important conclusions ; we see that 

 the force of impulsion driving ammonia into atmospheric air exceeds a pressure equiv- 

 alent to forty-three inches of mercury, the barometric pressure at the time being 29-73, 

 that is to say, exceeding by very near half an atmosphere the force which theory 

 would indicate. The hypothesis of Mr. Dalton, which seems to me to be fully con- 

 firmed by the observations of Mr. Thomson, founded on the experiments of Mr. Gra- 

 ham, assumes that gases act towards each other as vacua, or, in other words, the force 

 impelling the particles of one gas into the interstices of another does not exceed the 

 barometric pressure ; but here we find that the result apparently leads to a very differ- 

 ent conclusion. It was from an experiment of this kind that Dr. J. K. Mitchell was 

 led to doubt the truth of Dalton's theory, inferring from his results that gases penetrated 

 each other with much greater force. Such a conclusion, however, does not legitimately 



