PROFESSOR TYNDALL ON THE ACTION OF FREE MOLECULES ON 
a circular liquid layer of a given diameter could be vaporized in a tube of the same 
diameter, the absorption would remain unchanged. In other words, I thought that 
the liberation of the molecules from liquid cohesion would neither augment nor 
diminish their action upon radiant heat. Since 1864 this problem has been often in 
my mind. The wide silvered tube, I am happy to say, has rendered the solution of 
the problem possible. 
It is only highly volatile liquids that lend themselves to this experiment, because 
from them alone can vapours be derived of sufficient density to produce liquid layers 
of practicable thickness. On the 22nd of last October the experiment was first 
attempted. The source of heat was the lime light, the rays of which were received 
by a concave mirror silvered in front, and sent in a nearly parallel beam through the 
experimental tube. At the end nearest the source the tube was provided with a 
diaphragm having a circular orifice 1 inch in diameter. At the other end was a 
diaphragm with an orifice J an inch in diameter. Beyond this was placed the thermo¬ 
pile furnished, not with its reflecting cone, but with a tube of brass (shown in fig. 3) 
2 inches long, and blackened within. In this arrangement, the heat which reached 
the pile did not even approach the interior cylindrical surface. The total heat employed 
produced a deflection of 60 galvanometric degrees, which, when the tube was exhausted 
by a powerful Bianchi’s air-pump, was accurately neutralised by a compensating cube. 
Liquid sulphuric ether was then placed in a large flask provided with a sound stop¬ 
cock, the object being to expose a considerable evaporating surface. The flask was 
plunged in water, with the view of keeping the liquid and its vapour at an approxi¬ 
mately constant temperature. The air being carefully removed from the flask, it was 
attached to the experimental tube, and a quantity of vapour was allowed to enter 
sufficient to render a column 38 inches long equivalent to a liquid layer 1 millimeter in 
thickness. Two concurrent experiments made the deflection produced by the vapour 
41° 
Without altering the quality of the heat, the absorption exercised by a liquid layer of 
sulphuric ether 1 millimeter thick was next determined. The rocksalt cell with which 
the experiment was made is described in detail in the Bakerian Lecture for 1864 
(Phil. Trans., Vol. 154, p. 328.) The annexed figure (fig. 5) will give a sufficiently clear 
notion of its construction and disposition. Between two stout plates of brass, c c, and 
its fellow, two rocksalt plates of extreme purity are firmly clasped by suitable screws, 
due care being taken to protect the plates from a crashing pressure. The two brass 
plates referred to are perforated by circular apertures as shown in the figure. The two 
plates of salt are not allowed to come into contact but are separated from each other 
by a carefully worked brass plate 1 millimeter thick, perforated like c c. A portion of 
this middle plate is cut away, opening a passage into the interior of the cell. Through 
this passage the cell is filled with liquid by means of the funnel f. The cell is placed 
on a platform P P riveted on to the double brass screen S S. The source was placed 
at A, and the thermopile with its blackened tube at B. It is not necessary to figure 
them. 
