THE TEMPERATUEE OF PLANTS, AND OP THE GROUND. 
39 
those particles, the recipient of the surplus heat ; and if so, what is that etherial repellant 
element ? Let the philosopher reply. 
" Water is vaporisable at all temperatures. Its expulsion from the earth does, even 
under certain circumstances, continue when the atmosphere is replete with moisture ; 
and it is most important to observe that, however low a temperature the water in the soil 
or that of the atmosphere incumbent on it may be, at which vapour is formed and 
expelled, the same amount of heat is carried off hy a given weight of vapour, as if it had 
been generated in the open vessel over the fire, or in the close boiler of a high-pressure 
steam-engine." Here, before I proceed further on Mr. Parkes' authority, it may be 
interesting to notice the fact, that the vapour which impetuously rushes from the safety- 
valve of the said engine, imparts no sensation of heat to the hand, held at a few inches 
above it ; and indeed so long as the vapour shall remain invisible, and not condensed into 
visible steam. On the contrary, the feeling produced is rather that of a cool, forcible 
stream. But if the hand were raised above the invisible vapour, the flesh would be 
intensely scalded in a second of time. Vaporisation is attended with much electricity, 
and by that, perhaps, the watery particles are so repelled and reduced in size, as to 
become perfectly invisible, under which circumstances, the cool freshness of the electric 
stream alone is felt, for heat that cannot be, which produces cold. 
" It has been ascertained that it requires two or three ounces of coal to convert one 
pound of water into vapour ; therefore it is evident that an enormous quantity of heat must 
be taken from the soil in cases where water is allowed to remain stagnant until it 
evaporates." 
Parkes estimates the quantity of rain which may fall upon an acre in the course of 
one year, at thirty inches in perpendicular depth, or in volume, at 108,900 cubic feet, 
a sum which, divided by 365, will give 298 cubic feet — 18,0471bs. per day! This weight 
of water, he says, would require for its diurnal evaporation, supposing it could be so 
carried off — the combustion of 24 cwt. of coals, or I cwt. per hour per acre throughout the 
year ! Now, admitting the correctness of the above statement, we may imagine the vast 
depression of temperature which the earth must sustain if the excess of water over and 
above the due quantity required for vegetation, be carried off as vapour, at the expense of 
the heat, or of its agents, existing within the body of the soil ! 
Excess of moisture obstructs the absorption of heat by the solid matters of the ground, 
since it is proved by actual experiment that stagnant and quiescent water conducts little 
or no heat from the surface downwards. A very satisfactory article on this subject 
appeared in the Gardeners Chronicle of January 20th, with an appropriate figure, by 
which it was shown that a thermometer fixed horizontally at the bottom of a deep glass 
cylinder, previously filled, within an inch or two of the brim, with cold water, would not 
indicate any increase of temperature, when the remaining space was filled with boiling 
water, so cautiously added as not to create any disturbance in the body of the fluid. On 
the contrary, if a mass of water be heated from below, as it is in many familiar operations, 
the temperature of the whole acquires an equable temperature. The lower heated 
particles rise rapidly, their place being supplied by the colder and heavier particles which 
descend ; and thus a circulating motion is created, and increases, till the whole volume 
attains the boiling point. 
{To he continued.) 
