G.— ENGINEERING. 123 



sands formed of decomposed granites or in the porous structures of bricks 

 and some rocks. I have known water rise up within a glass tube H inches 

 in diameter filled with fine sand, composed of decomposed granite (the 

 lower end of which was immersed in a dish containing i inch of water) 

 6 or 7 feet in a few weeks. This, at first glance, might seem of little moment 

 from a practical engineering point of view, but when subsoils having this 

 characteristic have to be drained, so that trenches may be safely sunk 

 therein, the effect of the included water on materials is a serious matter 

 in the construction of engineering works. The lubricating effect set up 

 by the water produces quicksands out of the very fine particles which, 

 when dry, are relatively easy to handle by ordinary trenching methods. 

 The included water makes a semi-fluid mass which becomes extraordinarily 

 difficult to deal with in works involving excavation because the subsoil 

 refuses to give up the water it contains, and will flow through slight 

 crevices and under driven piling sunk into it many feet. 



In connection with mixtures of concrete, voids play a very important 

 part in the creation of what is practically artificial conglomerate. Further, 

 the minimum size and the variety of sizes forming the inert parts of concrete 

 mixtures make a great difEerence to the structure arising out of a mixture 

 of sand and stone (commonly called ' the aggregate ') with a varying 

 amount of cement. It is well known that the size of the cement particles 

 themselves make a difference to the necessary hydration, which reunites 

 them after their divorcement by heat and subsequent grinding to a very 

 fine powder. In the case of airtight or watertight concrete it is a matter 

 of great importance that all interstices be filled with cement when it has 

 become hydrated. It forms the adhesive matrix between the other 

 materials, which economise by dilution and form the greater part of the 

 mass. 



It is well known that an infinite number of different sized perfect 

 spheres would ultimately create a mass with no voids at all, if their variety 

 of size be carried to an impossible extreme. In many structures it is an 

 ideal condition of things to get as near as may be practicable to this state 

 of affairs when, as for example, an impervious concrete for dams and tanks 

 is desired. 



Doubtless many of you know what is commonly called the sand lime 

 brick largely made in Switzerland from glacial moraine in the rivers, where 

 stones of many different categories and sizes are brought down by the 

 melting ice-fed rivers into the valleys. I have seen excellent bricks made 

 in the Aar Valley out of a combination of a large number of different sizes 

 down to m.inute particles of crushed stone in which the remaining voids 

 were so small that only 2h per cent, of very finely ground lime to form the 

 matrix was all that was needed to make what was practically an artificial 

 solid stone. I have never heard of any other sand lime brick with such 

 a minute amount of lime matrix producing this effect. All this points 

 to the necessity of a minute analysis of sizes and shapes of particles to get 

 the most economic use of lime or Portland cement in the making of a mass 

 of artificial stone. Cement concrete differs only from masonry on general 

 broad lines. Masonry, in the form of concrete, can be put in with the 

 shovel, whereas hewn block masonry may involve lifting appliances and, 

 in any event, very careful handling and setting to produce the desired 



