114 On the Low Temperature qf Particular Caverns^ 
but that process, though capable of producing in certain cir- 
cumstances a great diminution of temperature, can contribute, 
in our opinion, very little to the effect under consideration. Air 
completely charged with humidity, at the temperature of 44° 
for example, the temperature of one of the caves of* Monte Tes- 
taceo must have possessed an extraordinary degree of dryness,, 
at the temperature of 78°, before it entered the vertical perfora- 
tions which conducted it to the caves, to allow it to be cooled at 
all by evaporation ; and if we assume, that, when it reached the 
cave, it had not been brought to a state of perfect dampness, 
this supposition would involve us in greater difficulties, as it would 
imply that the external air was in a state of dryness, which can 
rarely occur at Rome. At the temperature of 44°, a cubic incli 
of air is capable of holding, in the vaporous condition, .002082 
grains of Avater, and at the temperature of 78°, no less thati 
.005878 grains ; so that air, at the temperature of 78°, charged 
with the quantity of water it could hold in solution at 48°, Avould 
2032 
possess a relative humidity, represented by or .346, which 
Avould correspond to 23° of De Luc’s hygrometer,— a degree of 
dryness which is seldom observed, except in high latitudes. It 
would seem, therefore, that, to whatever cause the low tempera- 
ture is owing, wiiich prevails in the caves of Monte Testaceo, 
and the icy caverns formerly described, no part of the effect can 
be ascribed to evaporation. On the contrary, we have not the 
smallest doubt it will be found, by a more particular examina- 
tion of the state of those caves, that a deposition of moisture takes 
place from the external air before it reaches the cavern. At the 
same time, it is difficult to conceive in what other way the air 
could be reduced to a temperature so low as 44°, in the latitude 
of Rome, at a place so slightly elevated above the level of the 
sea, merely by passing through a quantity of loose materials, 
whose mean temperature cannot be less than 46 
In the case of the icy caverns, however, there is room for a 
wider range of conjecture. Among the various opinions which 
might be advanced to explain the cause of their depressed tem- 
* The mean temperature of Home is, according to Humboldt, 45°. 9 in winter, 
ana 55°. 2 in summer. 
