Matter is made up of atoms having dimensions approximately determined to be in the neighbourhood of the one fifty-millionth of an inch in diameter. These atoms may have various degrees of aggregation;--they may be in practical contact, as in most solid bodies such as metals and rocks; in molecular groupings as in water, and in gases such as hydrogen, oxygen, and so forth, where two, three, or more atoms cohere so strongly as to enable the molecules to act under ordinary circumstances like simple particles. Any or all of these molecules and atoms may be separated by any assignable distance from each other. Thus, in common air the molecules, though rapidly changing their positions, are on the average about two hundred and fifty times their own diameter apart. This is a distance relatively greater than the distance apart of the earth and the moon, for two hundred and fifty times the diameter of the earth will be 8000 × 250 = 2,000,000 miles, while the distance to the moon is but 240,000 miles. The sun is 93,000,000 miles from the earth, and the most of the bodies of the solar system are still more widely separated, Neptune being nearly 3000 millions of miles from the sun. As for the fixed stars, they are so far separated from us that, at the present rate of motion of the solar system in its drift through space--500 millions of miles in a year--it would take not less than 40,000 years to reach the nearest star among its neighbours, while for the more remote ones millions of years must be reckoned. The huge space separating these masses is practically devoid of matter; it is a vacuum. THE ETHER IS CONTINUOUS.