ell
to keep in mind that whenever the western continent extended to the
antarctic circle it prevented the independent circulation of the
Southern Ocean waters, consequently during such times ice periods
could not have occurred in the southern hemisphere.

It will be noticed that according to the views given above, the
several theories which have been published to account for great
climatic changes neglect to set forth the only efficacious methods
through which nature works for conveying and withdrawing tropical heat
sufficient to cause temperate and frigid periods in the high
latitudes. While lack of space forbids an explanation of the causes
which would perfect an ice period in the northern hemisphere, I will
say that it could be mainly brought about through the independent
circulation of the arctic waters, which now largely prevent the
tropical waters of the North Atlantic from entering the arctic seas,
thus causing the accumulation of ice sheets on Greenland. But before a
northern ice period can be perfected, it seems that it will need to
co-operate with a cold period in the southern hemisphere; and in order
to have the ice of a northern frigid period melt away, it would
require the assistance of a mild climate in the high southern
latitudes.--_Science_.

* * * * *

AMMONIA.

In the majority of refrigerating and ice machines ammonia gas is the
substance used for producing the refrigeration, although there are
other machines in which other material is employed, one of these being
anhydrous sulphurous acid, which is also a gas. Ammonia of itself is a
colorless gas, but little more than one half as heavy as air. In its
composition ammonia consists of two gases, nitrogen and hydrogen, in
the proportion by weight of one part nitrogen and three parts
hydrogen. The gas hydrogen is one of the constituents of water and is
highly inflammable in the presence of air or oxygen, while the other
component of ammonia, nitrogen, forms the bulk or about four-fifths of
the atmosphere. Nitrogen by itself is an inert gas, colorless and
uninflammable. Ammonia, although composed of more than three-fourths
its weight of hydrogen, is not inflammable in air, on account of its
combination with the nitrogen. This combination, it will be
understood, is not a simple mixture, but the two gases are chemically
combined, forming a new substance which has characteristics and
properties entirely different fr