s cases in which the more powerful waves are ineffectual,
while the more minute waves, through what may be called their
timeliness of application, are able to produce great effects. A
series of these, of a novel and beautiful character, discovered in
1868, and further illustrated in subsequent years, may be exhibited by
subjecting the vapours of volatile liquids to the action of
concentrated sunlight, or to the concentrated beam of the electric
light. Their investigation led up to the discourse on 'Dust and
Disease' which follows in this volume; and for this reason some
account of them is introduced here.

*****

A glass tube 3 feet long and 3 inches wide, which had been frequently
employed in my researches on radiant heat, was supported horizontally
on two stands. At one end of the tube was placed an electric lamp,
the height and position of both being so arranged, that the axis of
the tube, and that of the beam issuing from the lamp, were coincident.
In the first experiments the two ends of the tube were closed by
plates of rock-salt, and subsequently by plates of glass. For the
sake of distinction, I call this tube the experimental tube. It was
connected with an air-pump, and also with a series of drying and other
tubes used for the purification of the air.

A number of test-tubes, like F, fig. 2 (I have used at least fifty of
them), were converted into Woulf's flasks. Each of them was stopped
by a cork, through which passed two glass tubes: one of these tubes
(a) ended immediately below the cork, while the other (b) descended to
the bottom of the flask, being drawn out at its lower end to an
orifice about 0.03 of an inch in diameter. It was found necessary to
coat the cork carefully with cement. In the later experiments corks
of vulcanised India-rubber were invariably employed.

The little flask, thus formed, being partially filled with the liquid
whose vapour was to be examined, was introduced into the path of the
purified current Of air. The experimental tube being exhausted, and
the cock hick cut off the supply of purified air being cautiously
turned on, the air entered the flask through the tube b, and escaped
by the small orifice at the lower end of into the liquid. Through
this it bubbled, loading itself with vapour, after which the mixed air
and vapour, passing from the flask by the tube a, entered the
experimental tube, where they were subjected to the action of light.

The whole arrangement