With oxygen, hydrogen unites to form the important compound water, which exists not only in the obvious form of oceans, rivers, lakes, rains, dews, &c., &c., but is found intimately combined with many substances, giving them some of their peculiar properties. Many crystals have a definite proportion of water combined with them, and on losing this water they lose their crystalline form. Many acids also cannot exist as acids without water. The slaking of lime depends upon the union of water with the lime, the dry powder resulting from the process being a _hydrate_ of lime, the water having become _solidified_, and in passing from the fluid to the solid state gives out its latent caloric, producing the heat observed during the process. When a large quantity of lime, a barge-load for instance, has got wetted by accident, the heat evolved has been sufficient to set fire to the barge. At the temperature of 32° of Fahrenheit’s thermometer, water loses its fluid form, and becomes ice. As it solidifies, it starts into beautiful crystals, which unite and cross each other at determinate angles. Ice is lighter than the water on which it floats, forming a protection to the water beneath, and preventing it from being frozen so rapidly; else, if the ice were _heavier_ than water, and consequently sank as soon as formed, each portion of water would be frozen in its turn, until rivers became solid throughout, and every living creature in them must be destroyed. Now, the temperature of the water under the ice is seldom much below 40°, and if care be taken to break holes at intervals to allow access to the air, the fish and other aquatic animals seldom suffer even in our coldest winters. Although it is impossible to raise ice even one degree above 32° without thawing, it is not difficult to reduce water many degrees below that point without freezing it. In order to obtain both the constituents of water in a separate state, it must be decomposed by galvanism, each pole of a battery terminating in a separate tube containing water, when the result will be that at the positive pole oxygen gas will be evolved, and hydrogen at the negative, the latter being double the quantity of the former. Now, if you mix the gases thus obtained, introduce them into a vessel called a “Eudiometer,” and pass an electric spark through them from a Leyden vial, a sudden flash will be seen, and the gases will entirely disappear, being again converted into water. If you have a mercurial trough, and perform this experiment over mercury, the inside of the eudiometer will exhibit minute drops of water. Thus you have proved both by _analysis_ and _synthesis_, that water consists of oxygen and hydrogen, in the proportion of one volume of the former to two of the latter. Experiment.