For general work gravel or broken stone always is preferred to other coarse aggregate. Other materials at times are easier to obtain and, when used with discretion, will provide a satisfactory concrete. Broken terra cotta, brick, and old concrete, if hard and strong, may be used for unimportant work where no great strength is required, but special care should be taken that the particles do not show on the finished surface. The maxim that a chain is only as strong as its weakest link applies to concrete. If the coarse aggregate is weaker than the cement mortar, as in the case of some sandstones, it should be used with caution. The aggregate may have properties that render it unsuitable for use under certain conditions; for instance, cinders should not be used if water-tightness or strength is expected, but they are useful for fireproofing. Material that disintegrates or flakes when heated is undesirable in places exposed to high temperature; thus marble and some limestones should not be used in fireplaces. Some aggregates when exposed at the surface of concrete are apt to cause discolorations, and when this would be objectionable aggregates of this type should be avoided. Flat or elongated slab-like fragments should be avoided, as particles of this shape do not bond well; slate and shale are examples. CINDERS. Cinders should be composed of hard, clean, vitreous clinkers, free from sulphides, soot, and unburned coal or ashes. As a precaution against the presence of small amounts of detrimental substances, cinders should be soaked thoroughly with water 24 hours before being used. If clean they will not discolor the hands when a small quantity is rubbed between the palms. Cinder concrete, on account of its light weight, commonly is used for filling between sleepers of floors and grading roofs, and frequently for fireproofing, for which it is very effective. Cinders should never be used when the concrete is to be subjected to heavy loads or abrasion. LAVA ROCK. Lava rock varies widely in chemical composition and physical qualities. In some instances lavas are so light and frothy or contain so large a proportion of easily oxidizable material that they are wholly unsuited for concrete work. In general, the lava rock found in the Northwestern States is a suitable substitute for gravel. Rhyolite, a light colored volcanic rock, and many of the darker colored basaltic lavas can well be used for concrete for building purposes. WATER. Water should be clean and free from strong acid and alkali. Sea or brackish water should not be used if fresh water can be obtained. PROPORTIONING THE MATERIALS.