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April 22nd 1844
I thought to day of a process by which the force of tension inward of the bubble could be approximately obtained.  [[image]]
When a bubble is touched at its lower side to plate which has been moistened, it spreads out until it forms a perfect hemisphere.  The reason of this ^is that the attraction of the plate for the film draws it down and would cause it to cover the whole surface were it not that through the suction of the contained air, [[strikethrough]] and [[/strikethrough]] the contrast.  The force of the bubble comes in opposite to this attraction, and the two forces are evidently in equilibrium when the [[strikethrough]] perpendicular [[/strikethrough]] sides of the bubble are perpendicular to the surface of the plate, or when the bubble assumes a perfectly hemispherical form. While the bubble is spreading, if a [[strikethrough]]  thin [[/strikethrough]] film of water be poured on the plate, this will be drawn before the bubble, or if a sheet of water be placed on the plate, the water within the bubble will be found lower than on the outside by at least the 1/20 of an inch.  [[image]] [[in the margin]] For a paper on the strength of boilers, see Franklin Journal, vol 32, pg 54.  F=force of steam 
P=cohesive force
E=exchange
d=diameter 
F= 2PE
   ---
    d
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The result of the last arrangement was not satisfactory.  I therefore made the following.  A glass tube was twice bent at right angles, one part of it having first been drawn out into a tube of of about the 20 of an inch in diameter. The other part was about 5/8ths of an inch. This inverted syphon was filled with soap water, and the liquid was observed to stand higher in the smaller leg.  Its position was accurately determined by means of a scale [[underlined]] d [[/underlined]] placed behind the small end of the tube, and a microscope with a glass of short focus placed before. the scale was one belonging to a set of math instruments, and the microscope one for reading off the [[circle]] vernier [[/circle]] of an astronomical quadrant.
The divisions of the scale were the 1/45 parts of one  
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[[upper left]] April 22nd 1844
[[upper right]] 115 
inch, and these divisions by means of the microscope, could readily be divided into four or more parts.  The ring of wire [[underlined]] E [[/underlined]] was first dipped into soap water, and thus furnished with a film of the liquid which was then passed over the end of the tube. Next, the blow-pipe (a common clay pipe), was charged with soap-water, and a bubble blown on the wire so that the tube might be open in the interior of the bubble.  When the bubble had attained the size of five or six inches, the height of the water was observed through the microscope, and this in all the experiments were observed to remain constant until the bubble became so thin at the top as no longer to sustain the contractile force of the [[strikethrough]] lower part, it therefore broke and at the instant of the rupture, the water in the smaller tube was seen to descend, and from the mean of a number of observations, I concluded that the descent was about 1/3 of one of the divisions or 1/45 X 1/3=1/135 of an inch.
This depression of the water is the measure of the contractile force of the sides of the bubble.  It differs considerably from the estimate I gave with the other method which was [[circle]] visiated [[/circle]] by the attraction of water for the bottom of the disc causing more water to be expelled from under the bubble than was due to the tension of the air within.  I do not see any objection to the method I have here given, but in order to insure comparable results, it is necessary that the liquid in the larger leg should stand half an inch below the upper end of the tube, otherwise the form of the curve of capillarity will be altered, and the liquid in the smaller end be changed in altitude on this account.
The soap bubble is capable of illustrating several of the principles of capillarity.  If a piece of fine wire be bent into the form of a ring of 5 inches in diameter,
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