94 
lit; MARKS ON NAT ORE. 
Specific Gravity of Hot and Cold Water. —If a quantity of cold water 
be poured into a vessel, a thermometer being immersed in it, and a quantity 
of hot water be poured carefully over it, so as to prevent the fluids mixing by 
the agitation, it will be found that the hot water will float in the cold, the 
thermometer immersed in the cold water will not rise; nor will a thermometer 
immersed in the hot water poured over it fall. But if, by introducing a spoon 
into the vessel, and agitating the water, a mixture of the hot and cold be 
produced, the lower thermometer will immediately rise and the upper fall, 
and both will ultimately stand at the same temperature, intermediately be¬ 
tween their former indications. If, on the contrary, hot water be first poured 
into a vessel, a thermometer being immersed in it, and then cold water be care¬ 
fully poured upon the hot, so as to prevent such agitation as would cause the 
fluids to mix and a thermometer be also immersed in it, it will be immediately 
found that the lower temperature will fall, and the higher one will rise. In fact, 
the cold water descends through the hot by its superior gravity; but in this case 
the fluids, in passing through one another, become mixed, and the whole mass 
will take an intermediate temperature.— Lardn. Cyclop. Hydrostat. 
Process of Water Boiling. —The process by which water is boiled in a ves¬ 
sel, affords an example of the effects of a liquid expanding by heat. When fire 
is applied at the bottom of a kettle containing water, the stratum of water im¬ 
mediately in contact with the bottom, becoming heated, expands, and is conse¬ 
quently lighter, bulk for bulk, than the water above it. By the general princi¬ 
ples of hydrostatics it ascends, and the colder liquid, descending takes its place. 
This becoming heated, in its turn likewise ascends; and in this manner constant 
currents upwards and downwards are continued, so long as the fire continues to 
act on the bottom of the vessel. Thus every particle of the water in the vessel, 
in its turn, comes into contact with the bottom, and receives heat from it; and 
by the continuance of this process the temperature of the water is raised until it 
boils.— Ibid. 
Cause of Dew. —The discovery of Dufay remained a barren fact, until the 
attention of Dr. Wells was directed to the subject. He argued, that, as a clear 
and cloudless sky radiates little or no heat towards the surface of the earth, all 
objects placed on the surface, which are good radiators, must necessarily fall in 
temperature during the night, if they be in a situation in which they are not ex¬ 
posed to the radiation of other objects in their neighbourhood. Grass and other 
products of vegetation, are in genaral good radiators of heat. The vegetation 
which covers the ground in an open champaign country on a clear night, will, 
therefore, undergo a depression of temperature, because it will absorb less heat 
than it radiates. 
The vegetables which thus acquire a lower temperature than the atmosphere, 
reduce the air immediately contiguous to them to a temperature below satura¬ 
tion, and a proportionately copious condensation of vapour takes place, and a 
deposition of dew is formed on the leaves and flowers of all vegetables. In fact 
every object, in proportion as it is a good radiator receives a deposition of mois¬ 
ture. On the other hand, objects which are bad radiators are observed to be free 
from dew. Blades of grass sustain large pellucid dew drops, while the naked soil 
in their neighbourhood is free from them.— Ibid. 
Physical Constitution of the Moon. —The moon has no clouds, nor any 
other indication of an atmosphere. Were there any, it could not fail to be per 
