340 PROFESSOR TYNDALL OR THE ACTION OF FREE MOLECULES ON 
near one end of T T' was connected with a barometer tube and an air-pump. A T-piece 
at the other end was connected on the one side with a purifying apparatus (not shown), 
consisting of two U tubes, one containing fragments of Carrara marble wetted with 
caustic potash, the other containing fragments of glass wetted with sulphuric acid. 
Before entering these U tubes the air was freed from suspended matter by a plug of 
cotton wool. On the other side the T-piece was connected with a quill tube of glass 
bent into the shape of a U, in the two legs of which a coloured liquid stood at the same 
level. The liquid column when standing at the same level in both arms of the U was 
350 millimeters high in each, while the free leg of the U rose to a height of about 
500 millimeters above the surface of the liquid (shortened in the figure). The source 
of heat was the lime cylinder L, rendered incandescent by a flame of coal gas and 
oxygen. The rays from the lime cylinder were received by a concave mirror B. silvered 
in front, and sent by it in a convergent beam through the manometer tube. The focus 
of the beam was within the tube and near its most distant end. The gas and oxygen 
were supplied from gasholders specially constructed for these and similar experiments ; 
long and futile experience of gas from the public mains, or compressed in iron bottles, 
having shown independent gasholders which could be kept at an unalterable pressure 
to be essential. 
The experiments were conducted thus :—A test-tube t, plunged in water, held by 
the glass g, contained the liquid whose vapour was to be examined. Through a cork 
which stopped the test-tube passed a narrow tube of glass, ending in a small orifice 
near the bottom of the test-tube, and at a considerable depth below the surface of the 
liquid. To augment this depth, and to economise the liquid, the lower part of the test- 
tube was drawn out to half the diameter of its upper part. A second narrow tube 
passed also air-tight through the cork, and ended immediately beneath it. Both tubes 
were bent at a right angle above the cork. The manometric tube being exhausted, air 
freed from its carbonic acid, its moisture, and its suspended matter, was allowed to 
bubble through the liquid in the test-tube, and to pass thence into the manometric 
tube. To spare the oxygen in the gasholder it was cut off during the interval between 
two consecutive experiments, the coal gas being kept continually alight. When the 
manometric tube was filled, which was always accomplished through an orifice of fixed 
dimensions, the oxygen was turned on, and the cylinder was allowed to remain for one 
minute under the action of the intensified flame. During this time a double silver 
screen S intercepted the radiation. At the end of a minute this screen, which moved on 
a hinge, was withdrawn, the beam then passing through the mixed air and vapour. 
The liquid standing in the adjacent leg of the narrow U-tube was immediately depressed, 
that in the opposite leg being equally elevated. The rise of this latter column above 
its starting point, marked zero on a millimeter scale, was accurately measured. Double 
this rise gave the difference of level in the two legs of the U, and this “ water pressure ” 
expressed the augmentation of elastic force by the absorption of radiant heat. 
Here follow a certain number of the measurements which have been thus made. 
They do not comprise the whole of the substances examined, 
