330 MISTING OF PORTLAND < KMKNT. 



square inch. In the French Government specification the 

 residue is limited to 20 to 25 per cent, on a sieve of 

 .'>2.2")(i meshes per square inch; that is about what is 

 known as a 180 mesh, or 180 meshes each way along the 



side of tlir square inch. 



It is further stated in the German rules that the wire 

 must have a diameter equal to one-half the width of 

 opening. The sieves in general use vary in fineness 

 from 50 to 180 to the linear inch. 



A rough test for fineness used by the millers consists 

 in sieveing a certain volume of cement, determined by 

 filling a standard vessel. The residue from this quantity 

 is then measured in a vessel having a volume one-tenth of 

 the first and divided horizontally into ten equal parts, 

 each of which of course represents a percentage. 



173. Tensile Strength. Tensile tests may be made of 

 samples of neat cement or a mixture of cement and sand. 

 The test pieces are usually in the form of briquettes 

 similar to those shown on Fig. 163. These forms of 

 briquettes, or very similar ones, have come to be the standard 

 shapes, giving the most uniform and reliable results. 

 Various sizes are used, but by far the most usual is one 

 giving a breaking section of 1 sq. in. The preparing of a 

 set of these briquettes requires a certain amount of skill 

 and experience, and should not be intrusted to anyone 

 who has not had the necessary training. 



The briquettes are formed in metal moulds, either made 

 -singly or in sets of five or more. In gauging the sample, 

 a weighed quantity of cement is to be mixed with a 

 certain definite percentage of water either a prearranged 

 percentage or the percentage which has been found most 

 suitable for the particular cement in question. This can 

 be found by gradually adding water from a known 

 quantity until the cement has absorbed sufficient to work 

 it up into a thick paste, and to cause the water to rise to 

 the top of the pat when the trowel is drawn over it. The 

 cement and water should be thoroughly mixed and 

 worked together with a trowel, the water being added 

 gradually until the required consistency is reached. The 

 theoretical quantity of water required for chemical com- 

 bination is somewhere about 8 or 9 per cent., but a 



