12 Mr. Harvey on the Formation of Dew. 
The difference in the appearance of dew, when deposited on tin 
and on glass, is sufficiently remarkable to arrest attention, not only 
when the moisture remains uncrystallized, but also when it is frozen. 
In an example that occurred of the latter case, a decrease in the 
magnitudes of the frozen particles could be traced from its edge to 
the dry and unfrozen margin surrounding a parcel of wool, placed 
on the middle of the plate, as represented in fig. 13: the appear- 
ance of the frozen atoms partaking, in some degree, of the lustre of 
the tin. The parcel of wool, in the interval from nine P. m. to mid- 
night, gained four grains of moisture ; and from the last mentioned 
hour, to six the next morning, thirty-two grains ; thus gaining, in a 
double time, an eight fold quantity of moisture. The wool was frozen 
to the tin; and when the rays of the sun fell on the metallic surface, the 
crystalline particles became detached from it, and were readily col- 
lected together. The dew deposited on the glass presented an irregu- 
lar fibrous appearance, its colour partaking of the greenish hue of 
the crystal. The icy particles on the tin were first deposited as dew, 
and frozen before they had collected in sufficient numbers to run 
into each other, and form an uniform crystalline surface. But the 
dew on the glass being formed at an earlier period of the night, a 
sufficient quantity was deposited to cause the particles to mingle 
with each other, and thus to present to the action of the freezing tem- 
perature a wide spread surface of water. The unequal action of the 
glass, combined with the law which regulates the crystallization of 
water, communicated to the frozen surface of dew, the fibrous and 
irregular character represented in fig. 12. Soon after the solar rays 
had impinged on the glass, filaments of ice were detached from 
both its surfaces, that from the upper side being much the thickest. 
Plymouth, December 12, 1823. 
