74 
Proceedings of the Royal Soeiety 
Tor testing the surface tensions an indirect way of measuring the 
tension of the surface of water in a vessel was adopted. This was 
done by finding the amount of pull necessary to raise a flat disc 
off the surface of the water. The disc in rising brings with it a 
certain quantity of water, and the amount of water lifted depends 
on the height to which the disc can he lifted above the surface 
before the film yields, and, as this depends on the strength of the 
film, the weight of water lifted forms an indirect way of measuring 
the strength of the film. The apparatus used consisted of a cir- 
cular disc 4-5 cm. diameter. In order that this disc might hang 
horizontally as nearly as possible, it was accurately turned on the 
end of a metal rod. The upper end of this rod was hung to one 
end of the beam of a balance. To the other end of the beam was 
hung a glass tube, inside which was placed a scale with the zero 
point near the top of the tube. A tall narrow vessel full of water 
was placed underneath, and so that the tube hung in the water. 
The tube was loaded with mercury till it just balanced the metal 
disc with the beam of the balance horizontal, and the tube im- 
mersed in the water up to the zero point. In order to get good 
contact between the water and the metal disc, a coating of plain 
Collodion was put on the disc. After all the collodion solvents 
had escaped, the apparatus was ready for a test. The vessel with 
water to be tested was placed under the disc and at the correct 
height, the beam, when the disc touched the water, being slightly 
inclined downwards at the disc end. The tall vessel of water was 
now slowly lowered. As the level of the water gradually went 
down, the weight of the graduated tube increased. While the 
level of the water descended, a careful watch was kept to observe 
at what number on the scale the disc broke away from the water. 
The object of using the submerged tube was that it allows of an 
increase of weight being put on very gradually and without shock. 
With this instrument the different readings for the same sample 
of water always agreed easily to 1 per cent., though they varied 3 or 4 
per cent, for different samples if the vessel which held the water was 
not carefully cleaned. 
Owing to the principle of the construction of this instrument, it 
does not give absolute readings, nor do the readings of the scale repre- 
sent numerically the relative strengths of the different films, so that 
