28 Mr. W. S. Jevons on the Cirrous form of Cloud. 
We must carefully distinguish this gradual filtration from the 
violent motions of large masses of fluid, also produced by differ- 
ences of density and pressure, but which are on so much larger 
a scale as to produce entirely different appearances. If the 
whole upper stratum in our first experiment were suddenly to 
become of greater density than the rest of the liquid beneath, it 
would undoubtedly sink down through the latter im large rapid 
streams ; but instead of this it is only small quantities of liquid 
at the junction of the strata, mere pins’ heads in size, which, 
from the equalization of temperature, become successively of suf- 
ficient density to sink, and they then do move with all the violence 
of which they are capable. 
It is the violent upward motion of currents of moist air which 
produces the cumulus, the rarefaction and reduction of tempera- 
ture, however, occasioning the precipitation of watery particles ; 
but just as cumuli rise in detached rounded masses separated by 
portions of clear air, which must have more or less of a descend- 
ing motion, so very small portions of air will filter upwards in 
distinct fine threads, with intervening threads moving in the 
opposite direction. 
But we are not wanting in some analogous and easily observed 
appearances in the air itself, as in the long wavy threads or bands 
of smoke rising: from the wick of a candle, or the light-streaks 
so distinctly seen moving across a sunbeam entering a room of 
which the air is smoky or thick. The chemist, too, cannot help 
remembering the thread-like clouds which always appear when 
hydrochloric acid and ammonia are exposed within the reach of 
each other’s vapour. By carefully employing these chemicals, a 
true and distinct cirrous cloud might no doubt be produced ; 
but gases are so easily disturbed and move so rapidly, as soon to 
cause complete mixture and confusion in any experiment on a 
small scale. 
On this theory, then, cirrous fibres are considered to be thread- 
like streamlets of moist air, forcing their way by the effect of 
gravity alone through masses of air likewise moist, but of differ- 
ent temperature and density; watery particles being of course 
precipitated, according to Dr. Hutton’s theory, and becoming 
visible over the surfaces of the streamlets. 
It will now be well to mention a few points in which the ana- 
logy between the miniature experiment and the real atmospheric 
cirrous cloud evidently fails. 
1st. The white precipitate of chloride of silver used to repre- 
sent the watery particles of a cloud is not resoluble in fresh quan- 
tities of the fluid; but any portions of watery cloud-matter sub- 
siding into or otherwise coming into contact with undersaturated 
air will immediately re-evaporate and vanish, so that appearances 
will be considerably modified. 
