bricks was 555 pounds and 16,675 pounds, respectively. For a brick measuring 8 inches in length by 4 in width, this would mean a crushing strength for the whole brick of from 21,152 to 443,936 pounds.

Transverse strength. This is more important even than the crushing strength, for while a brick is rarely loaded to its crushing limit, it is sometimes exposed to its limit of elasticity and cracked. This can perhaps be better understood if the manner of making the test is first explained. In the cross-breaking test a whole brick was placed on two rounded knife-edge bearings (Fig. 37), which in the tests made on the New Jersey brick were 6 inches apart. Pressure was then applied from above, as in

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Fig. 37.

Diagram showing method of testing the breaking strength of brick.

dicated by the arrow, until the brick broke in two, and the number of pounds pressure at which this occurred was noted. It is evident that in two bricks of exactly the same degree of strength, the amount of pressure necessary to break them will depend upon, 1) the distance between the supports, and 2) the cross section of the brick. The farther apart the supports the less the pressure necessary to break the brick, and the greater the cross section, the greater the pressure necessary. Since this is so; it is necessary that for purposes of comparison all results of the breaking strength be reduced to some uniform expression, which shall take account of the differences in length, width and thickness of the brick. The most accurate expression is that

termed the modulus of rupture, which is calculated from the following formula:

in which R= Modulus of Rupture.

W Pressure necessary to break the brick.

1= Distance between the supporting knife-edges.

b Breadth of the brick.

h

That is, three times the pressure in pounds multiplied by the distance between the supports is divided by twice the breadth of the brick, multiplied by the square of the thickness. If the pressure necessary to break a brick was 2,000 pounds, distance between supports 6 inches, width of brick 4 inches, and thickness 2 inches, the modulus of rupture would be calculated as follows:

Absorption tests.-An absorption test is made for determining how much water a brick is capable of absorbing. This indicates the degree of porosity which it possesses. Vitrified brick which are impervious, or nearly so, will absorb little or no water. Many common brick may absorb an amount of water equal to 15 per cent., or even 20 per cent. of their dry weight. It is easily understood that if a brick of high porosity is exposed to a freezing temperature when its pores are filled with water, the expansion of the latter, when changing to ice, may be sufficient to disrupt the brick, either after one freezing, or after repeating freezings following periods of thawing. A moderate or low absorption is therefore desirable.

In making the absorption determinations on New Jersey brick, the test was carried out on a half brick. The advantage of doing this is that the surface of the brick is sometimes slightly denser than the interior and acts as a protective skin against percolating water. If, therefore, a whole brick is tested, we do not always get its true absorptive capacity. In use, this outer skin may get worn off wholly, or in part, especially if the brick is exposed to abrasive action, as in pavements.

The samples tested were first thoroughly dried, then weighed, and soaked in water for 48 hours, after which they were weighed again. The increase in weight gave the amount of water absorbed, and this, divided by the original weight, gave the percentage of water absorbed. The percentage of absorption is, therefore, based upon the weight of the dried brick.

The absorption on 8 different kinds of soft-mud bricks was determined. The lowest of these was 5.36 per cent. and the highest 18.64 per cent., but 6 ranged between 12.09 per cent and 14.87 per cent. The average of the 8 was 13.39 per cent., or, omitting the two extremes, it was 13.86 per cent. Curiously enough, the highest and lowest figures belong to bricks from the same yard, but represent two different banks of clay. The one with the high absorption was red, but lacked in hardness; the other was hard, but its color was yellowish white with iron specks, and on account of its poorer color it did not command so good a market. The effect of re-pressing is seen in the lower absorption of brick from Trenton in the series given below.

Absorption tests on 14 kinds of stiff-mud brick from 14 localities ranged from 1.34 per cent. to 14.29 per cent., with an average of 10.19 per cent., which is somewhat lower than the absorption of the soft-mud samples. The following are the figures obtained :

The absorption tests are given separately because they include some samples which were not subjected to crushing tests.

Explanation of table. The following table gives the crushing and transverse tests made on New Jersey bricks. The first column gives the number of the brick, corresponding to the records kept in the Survey office. Each manufacturer has been informed which number represents his brick, and no firm names are published, as some brickmakers preferred not to have this done for business reasons, and their wishes are respected. The second column gives the character of material, and the third column gives the methods of manufacture, although aside from this the bricks are separated into the stiff-mud, soft-mud, and dry-press groups. In the fourth column the color of the brick is given, and in the fifth the character of the product. In the sixth, seventh, eighth and ninth columns there are data referring to the crushing tests. The sixth column gives the lowest test of those made, the seventh the highest, the eighth the average, and the ninth the number of bricks tested. In the next three columns the transverse tests are shown, the minimum, maximum and average being given, as in the crushing tests. The fourteenth represents the percentage of absorption, and the fifteenth, or last, column indicates the geological formation from which the clay or mixture of clays was obtained.