-------------------------------------------------- TESTS OF FIRE-CLAY AND SHALES.--PAVING BRICKS. ----------+-----------+---------+----------------- |Absorption.|Rattling.| Crushing. +-----------+---------+--------+-------- | | |Square | Cubic | | |Inches. | Inches. | | +--------+-------- Shales | 1.17 | 17.61 | 7,307 | 1,764 Fire-clay | 1.62 | 17.32 | 6,876 | 1,678 Mixture | 1.44 | 18.72 | 5,788 | 1,400 River Clay| 1.36 | 19.02 | 4,605 | 1,176 ----------+-----------+---------+--------+-------- From a series of tests recently made by Mr. Fickes,[22] the following factors were educed:-- 1. A brick which stands the “rattling” test well, has ample crushing strength and rarely chips under less than 5,000 lbs. per square inch, or crushes under less than 10,000 lbs. The crushing strength tends to vary with the resistance to abrasion, however, but more slowly and irregularly. 2. The transverse strength also tends to vary with the resistance to abrasion, but more slowly and irregularly. 3. The toughest bricks usually absorb the least water. _Specific Gravity._--The practical value of knowing the specific gravity of a brick has, perhaps, been a little over-rated by writers on the subject. At the same time we do not deny that there is some use in ascertaining this property. Foremost, we have to mention its value in conjunction with absorption in arriving at a rough and ready means of gauging the strength of a brick, without having actual recourse to the crushing machine. It appears to us, however, that the specific gravity of bricks is rarely quoted in a proper manner, and until there is one uniform method, the results will always be at a discount. We allude to the fact that some experimenters take the specific gravity of a porous brick, without stating whether the amount of water absorbed, during the process, was taken into account in arriving at the specific gravity or not. Theoretically, of course, the substance to be dealt with is non-porous, and experimenters, worthy the name, either render the brick waterproof, or, ascertaining the amount of water the brick has absorbed, take that into consideration in calculating results. The writer is in the habit of quoting the specific gravity in two ways, viz.: (_a_) the true specific gravity, and (_b_) the specific gravity of the particles. In an elementary treatise like the present, however, it is not desirable to enlarge on this subject. THE END. FOOTNOTES [1] This, and all other technical terms used, will be explained in an alphabetical glossary at the end of the book. [2] “Canal and River Engineering,” p. 315. [3] See, Geikie’s “Text Book of Geology,” 1882, p. 72. [4] Information on this subject will be found in Mr. J. H. Collins’ work, “The Hensbarrow Granite District.” Truro, 1878.