more than a bottle from which all air has been exhausted. Air weighs seven one-hundredths of a pound to the cubic foot. It would take 10,000 cubic feet of air to weigh as much as one cubic foot of lead.

We have before noticed that the closer together the atoms of matter are packed, the greater the cohesion. This is also true of gravity acting on matter. The closer inatter is brought to the centre of the earth, the more it is attracted by gravity. Gravity acts with much greater force at the bottom of a deep mine than at the top of a high mountain.

We have thus far considered only a few of the properties of matter, but they have been very important properties, a knowledge of which is necessary to the intelligent exercise of ordinary every-day operation with materials. Other properties of matter will be spoken of further along in these pages. We have looked into the structure of matter, and find that the peculiar arrangement of molecules in different substances gives to those substances different qualities of weight, hardness and strength.

The properties of almost all substances have been determined by practical scientific investigation, and it is thought well to insert at this point tables showing the different relative properties of different well-known substances.

WEIGHT OF DIFFERENT SUBSTANCES.

The following table shows the weight in pounds of a cubic foot of various common substances. From this table we can calculate the weight of any mass of any substance in the table, by first ascertaining by measurement and calculation how many cubic feet there are in the mass, and multiplying the result by the weight of a cubic foot of the substance:

STRENGTH OF SUBSTANCES.

The following table shows the breaking strength of, or power in pounds required to pull apart a bar one inch square of various common substances. The strength of any piece of substance shown in the table can be found by measuring or calculating the number of square inches

area of the piece and multiplying the result by the breaking strength of one square inch, as shown in the table:

HARDNESS OF SUBSTANCES.

It is, of course, impossible to speak of hardness in pounds, or square feet, or anything of that kind. We can only compare the hardness of one substance with the hardness of another substance. Thus, we can say that chalk is half as hard as gold. It is best to take some one substance as a standard and compare other substances with it. Taking Ormuz diamond as the standard, we find cork to be one-twentieth as hard. For convenience it is best to give the standard a value of one hundred, and express the hardness of other substances in hundredths of the standard. The following table shows the relative hardness of various substances, Ormuzdiamond being taken as the standard :