230 
CHEMISTRY OF HORTICULTURE. 
breath of life. Air, it is true, can be rent and torn to pieces by powerful electric agencies, 
but then it is no longer air ; its gases become separated, and in their act of separation 
produces other combinations. Connected with this subject, I find a passage in the Penny 
Cyclopaedia, which merits insertion: — 
*' It has been found that two gases in a state of mixture exercise no influence one upon 
the other except communication of temperature, but that each is disposed in exactly the 
same manner as it would be if the other were not present. Thus it is found, entirely 
contrary to all previous notions, that no pressure of dry air upon water exerts the least 
influence in preventing the formation of steam, which goes on exactly as if the space above 
were a vacuum, and continues until further evaporation is stopped by the pressure of the 
steam already created. It is also found, that no degree of pressure of one gas can confine 
another in water ; but that, supposing a bottle partly full of water, the gas confined in the 
water will escape to the surface and distribute itself in precisely the same way as if other gases 
were not present. By this it is not meant that the action, commonly called mechanical, 
cannot take place, or that a stream of hydrogen would not trouble the air ; but only that 
the permanence of one gas is not affected in any way by the presence of another, so long 
as no chemical action is excited." 
At this point, I think, we find some reason to conjecture that the theory of mechanical 
mixture may not be tenable, and for the precise reason that is advanced in its favour. 
Mr. Dalton taking into consideration the presumptions which exist against the chemical 
union of the ingredients of the atmosphere, {i. e. of the oxygen and nitrogen,) infers that 
the atmosphere does not consist altogether of the compound called air, but that the nitrogen 
atmosphere is higher than the oxygen atmosphere. 
"In fact, if there is no mechanical union, the above law of the mixture of the gases 
requires us to allow that each is an atmosphere independent of the other, and that the two 
are, most probably, of two unequal heights." 
But what say facts ? Did not Gay Lussac bring down air from a height of more than 
four miles, which was not found to differ from that of the earth's surface in the proportion 
of its oxygen to its nitrogen ? Has any circumstance, or degree of any experimental analysis 
ever, in one instance, led to the inference that the two gases exist in any condition other 
than that admitted by all to be indisputably proved ? Now, we maintain that the adven- 
titious gases, not only those which have been already alluded to, but others which degrade 
and contaminate the air, producing ague and epidemics, do exist in a temporary state of 
mechanical union only, till they are either brought down by rain, or act upon each other 
electrically, producing true chemical results, but yet (providentially) without being able to 
disturb the perfect union of the two basal gases, in their mixed proportions of about 77^ 
nitrogen to 21 of oxygen. 
Atmospheric vapour is the source of many beautiful and interesting phenomena. Nothing 
can be more so than the sudden absorption and disappearance of frosty rime. The readers, 
some at least, may probably follow us when we recal to mind that often, in winter, the 
trees, bushes, and herbage are completely encrusted with frosty spiculas, so that each twig 
or stalk becomes enlarged to double its usual thickness. 
A mist, or diffused stratus, in the early morning, the temperature being below the 
freezing point, is generally the cause of this deposition. The mist then disperses, 
ascends and forms clouds of varying modifications ; gradually, however, (the frost still con- 
tinuing,) the frosty rain vanishes, no drops of water fall, it is taken up and absorbed by the 
air, and in an hour or two, not a particle of hoar frost remains on herbage or plants. On 
one occasion, noted by me particularly, the rime coated all vegetation, the temperature at 
8 o'clock being about 16°; there was no wind, no material difference in temperature, yet 
without any sensible cause, without thaw or solution, the whole of the rime vanished, every 
particle disappeared, it was all taken up a short period before noon, and passed into the 
atmosphere as invisible vapour, the transparency of the air was, however, lost and clouds 
formed. At from 2 to 3 o'clock the thermometer rose to 24°, by night-fall to 30®, 
on the following morning to 40°, and then rain commenced. 
The formation of clouds depends mainly upon the electric condition of the vapours ; and 
