ihere is scaruuij a, uuu, "" compact it may appear, wMch does not contain pores, and these pores are filled with air. It is to be foundjin abundance in the soil; indeed wcre it not so, numberless worms and insects which inhabit the latter, wonld cease to exist. The most compact mortar and walls are penetrated by it, and water in its natural state contains a large quantity of air in solution, The atmosphere was forruerly believed to extend no highor than five miles above the earth's surface, but meteorological observations, have since shown that it extends to a height of more than 200 miles. Owing to the force of gravity the air is much dcnser near tne earm, aun K" """"=!, jo' by layer, as you ascend. If, then, the atmosphere were possossod of color, it wonld be very dark just round the globe, and the tint would gradually fade into space. There is no absolutely normal composition of the air we breathe, or, if there be, it is not at present known. It contains, however, in all cases, unloss under puroly artificial conditions, two essential elemente, which are nearly invariable under normal circumstances, namelv oxygen and nitrogen, and two accessöry lements which vary oxtremely in amount, but are practicaüy never absent, namely carbonic acid an 1 water. Without either of the first two air could not exist, and without tho last two air is scarcely found in nature. There bmauon, muLuuïüi, m lw uui umon, but a simple mechanical mixture. But besidos theio constituents, the air contains an immense amount of small partióles derived from the whole creation. In the air may be found animálculos, sporos, seeds, cells of all kinds, eggs of insects, fungi, and elemonts of contagión, besides formless dust, and sandy and other partióles of local origin. For exaniple, no one can travel in a railway carriage without being surrounded by dust, a large portion of which may be attracted by a magnct, consisling, as it doea, in a great measure; of minuto partióles of iron derived from the rails. The purest air has sorue dust in it. There probably never feil a beain of light from the sun since the world was made which would not have shown nnnntless numbors of solid partióles.