in early spring. The expansion of water during its congelation, at which time its volume increases one twelfth, and its contraction in bulk during a thaw, tend to pulverize the soil, to separate its parts from each other, and to make it more permeable to the influence of the air. When ice changes to water, or water to steam, although at an invariable degree of temperature, yet the change is not sudden, but gradual. When the heat reaches 131 the point, at which thawing or boiling takes place, the temperature makes a stand ; a portion of it disappears, or becomes latent, as it is called ; thus the temperature of ice cannot be raised, till the whole is thawed, nor that of boiling water, till it has all been converted into steam ; all the heat that is applied being absorbed in producing these changes. Were it not for this law of latent heat, thaw aud evaporation would be instantaneous, we should be overwhelmed with floods, at the first glow of warmth in the spring, and in heating water the whole would flash instantaneously into steam upon reaching the boiling point. It is through the same relations of water to heat, that springs are supplied — for these undoubtedly draw their principal supplies from rain. Mr. Dalton has calcu- lated that the quantity of rain which falls in England is 36 inches a year. Of this he reckoned that 13 inches flow off to the sea by the rivers, and that the remaining 23 inches are raised again from the ground by evaporation. The 13 inches of water are of course supplied by evaporation from the sea, and are carried back to the land through the atmosphere. Vapor is perpetually rising from the ocean, and is condensed by cold in the hills and high lands, as is easily recognized by the mists and rains, which are frequent in such regions ; whence it descends through their pores and crevices, till it is deflected, collected and conducted out to the sea, by some stratum or channel which is water-tight, thus keeping up a perpetual and compound circulation. In every country these two portions of the aqueous circula- tion have their regular and nearly constant proportion ; and their due distribution appears to be necessary to its organic health, to the habits of vegetables and of man. This circulation goes on from year to year as regularly as that of the blood, in the veins and arteries of the human system, and though maintained by a very different machinery, is no less clearly adapted to its purposes. In short the properties of water which regard heat make one vast watering engine of the atmosphere, (Whe- well.) Common Water. Under this head are included the waters commonly known as rain, spring, river, well or pump, lake and marsh waters. Thomson includes ice, and snow water, spring and river water, and lake water under rain water, as it is from this source that they are chiefly supplied. Rain Water is the purest kind of all natural waters, though subject to some variations. Thus, when collected in large towns or cities, it is less pure than when obtained in the country ; moreover it is usually loaded with impurities at the com- mencement of a shower, but after some hours of continuous rain it becomes nearly pure ; for the first water which falls brings down the various foreign matters sus- pended in the atmosphere. In specific gravity, it scarcely differs from distilled water. It nevertheless generally holds in solution common air, carbonic acid, carbonate of 132 lime, chloride of lime, and a trace of nitric acid. If it be collected from the roofs of houses, after it has rained for some time, it contains sulphate of lime and occa- sionally carbonate of lead. The quantity of common air in rain water does not ex- ceed 3| cubic inches in 100 cubic inches of water ; it contains more oxygen than atmospherical air ; the same quantity of rain water contains one inch of carbonic acid gas. These combinations, in the small quantities in which they exist, in no degree in- jure the diluent properties of rain water. It is indeed to the presence of the two elastic gases, that rain water owes the taste which renders it palatable to animals and useful to vegetables. Ice water, being destitute of these gases is extremely vapid ; fish cannot live in it ; and it does not seem either to quench thirst or to be so complete a solvent in the stomach as rain water. Carbonate of ammonia is also another ingredient. It is derived from the putrefaction of nitrogenous substances. When several hundred pounds of rain water were distilled by Liebig, in a copper