A. Because straw (being a non-conductor) prevents the sap of the tree from being frozen.
Q. Can water be frozen in any way besides by frosty weather?
A. Yes; in very many ways. For example—a bottle of water wrapped in cotton, and frequently wetted with ether, will soon freeze.
Q. Why would water freeze if the bottle were kept constantly wetted with ether?
A. Because evaporation would carry off the heat of the water, and reduce it to freezing point.
Q. Why does ether freeze under the receiver of an air-pump, when the air is exhausted?
A. Because evaporation is very greatly increased by the diminution of atmospheric pressure; and the ether freezes by evaporation.
FREEZING MIXTURES.
1. If nitre be dissolved in water, the heat of the liquid will be reduced 16 degrees.