ORE TESTS 83 



a. Measure out J4 of a cupful of fresh, powdered cement, ^ 

 of a cupful of fine, sharp sand, and a cupful of rather fine gravel 

 (stones about Y^ the size of a coffee bean). Do the mixing 

 in a quart tin can (tomato can) ; for the stirring use a wooden 

 paddle or an iron spoon. First put in the sand; then add 

 the cement, and stir the two together. Wet the gravel, and 

 mix it with the sand and cement, adding water little by little, 

 so as to form a thick, "mushy" mixture. Finally pack the 

 mixture (use a block for a "tamp") into a small pasteboard 

 box; have the top of the mixture just below the top of the 

 box. 



b. Make a finishing coat for the concrete out of about l /& of a 

 cupful of cement and J4 of a cupful of fine sand. Put these into 

 the empty can, mix them with enough water to form a paste, 

 and spread the paste evenly over the concrete. Use a flat 

 block or a kitchen knife for a trowel. 



Cover the concrete loosely with paper, and set it aside for 

 3 or 4 days to harden. Note its appearance from day to day. 

 What is its color when hard? Finally remove the cover, expose 

 the concrete to the light, and wet its surface once or twice a 

 day. What is the effect of this treatment upon the color of 

 the concrete? 



c. What advantage has concrete over flagstones for use in 

 sidewalks? Over bricks? What is reinforced concrete? 



The mixture described in a is called a 1, 2, 4 mixture; why? 



EXERCISE 81 

 ORE TESTS 



Apparatus and Materials. Pieces of pyrite (iron pyrites), hematite, 

 galena, and copper ore; -magnet or magnetized knife blade; piece of 

 charcoal or of soft earthenware, such as a piece of a flower pot; mouth 

 blowpipe; Bunsen burner or alcohol lamp. 



