Fuly 6, 1871] 
shown, presents the apparent anomaly of being lower in 
the thermometric scale than its melting point, a statement 
less paradoxical than it may at first appear, if we remember 
that water can exist as vapour at temperatures far lower 
than those at which it can exist as liquid. Whether the 
transition be directly from solid to gaseous, or from solid 
to liquid and from liquid to gaseous, a marked change of 
physical properties occurs at each step or break, and heat | 
is absorbed, as was proved long ago by Black, without 
producing elevation of temperature. Many solids and 
liquids will for this reason maintain a low temperature, 
even when surrounded by a white hot atmosphere, and 
the remarkable experiment of solidifying water and even | 
mercury on a red hot plate, finds thus an easy explana- 
tion. The term spheroidal state, when applied to water 
floating on a cushion of vapour over a red hot plate, is, 
however, apt to mislead. The water is not here in any 
peculiar state. It is simply water evaporating rapidly at 
a few degrees below its boiling point, and all its pro- 
perties, even those of capillarity, are the properties of 
ordinary water at 96°°5C. The interesting phenomena | 
NATURE 
187 

exhibited under these conditions are due to other causes, 
and not to any new or peculiar state of the liquid itself. 
The fine researches of Dalton upon vapours, and the 
memorable discovery by Faraday of the liquefaction of 
gases by pressure alone, finished the work which Black 
had begun. Our knowledge of the conditions under 
which matter passes abruptly from the gaseous to the 
liquid and from the liquid to the solid state may now be 
regarded as almost complete. 
“Tn 1822 Cagniard de la Tour made some remarkable 
experiments, which still bear his name, and which may be 
regarded as the starting point of the investigations which 
form the chief subject of this address. Cagniard de la 
Tour’s first experiments were made in a small Papin’s 
digester constructed from the thick end of a gun barrel, 
into which he introduced a little alcohol and also a small 
quartz ball, and firmly closed the whole. On heating the 
gun barrel with its contents over an open fire, and observ- 
ing from time to time the sound produced by the ball 
when the apparatus was shaken, he inferred that after a 
certain temperature was attained the liquid had disap- 

Fic. 1—Cloud below critical point 
peared. He afterwards succeeded in repeating the ex- 
periment in glass tubes, and arrived at the following 
results. An hermetically sealed glass tube, containing 
sufficient alcohol to occupy two-fifths of its capacity, was 
gradually heated, when the liquid was seen to dilate, and 
its mobility at the same time to become gradually greater. 
After attaining to nearly twice its original volume, the 
liquid completely disappeared, and was converted into a 
vapour so transparent that the tube appeared to be quite 
empty. On allowing the tube to cool, a very thick cloud 
was formed, after which the liquid reappeared in its former 
state. 
“Ttis singular that inthis otherwise accurate description 
Cagniard de la Tour should have overlooked the most 
remarkable phenomenon of all—the moving or flickering 
strice which fill the tube, when, after heating it above the 
critical point, the temperature is quickly lowered. This 
phenomenon was first observed by the lecturer in 1863, 
Fic. 2—Strie above critical point 
appearances exhibited by the ascending and descending 
sheets of matter of unequal density are most remarkable, 
but it is difficult to give an adequate description of them 
in words or even to delineate them. 
“These strize arise from the great changes of density 
| which slight variations of temperature or pressure pro- 
when experimenting with carbonic acid, and may be ad- | 
mirably seen by heating such liquids as ether or sulphu- 
rous acid in hermetically sealed tubes, of which when 
cold they occupy about one-third of the capacity. The 
| 
duce when liquids are heated in a confined space above 
the critical point already referred to; but they are not 
formed if the temperature and pressure are kept steady. 
When seen they are always a proof that the matter in the 
tube is homogeneous, and that we have not liquid and 
gas in presence of one another. They are, in short, an 
extraordinary development of the movements observed in 
ordinary liquids and gases when heated from below. The 
fact that at a temperature o°'2 above its critical point 
carbonic acid diminishes to one-half its volume from an 
increase of only s, of the entire pressure is sufficient to 
account for the marked characters they exhibit. 
“Tf the temperature is allowed to fall a little below the 
critical point, the formation of cloud shows that we hive 
now heterogeneous matter in the tube, minute drops of 
liquid in presence of a gas. From the midst of this cloud 
