go THE WOMBAT. 



and of the water used, and the speed with which the bri- 

 quettes are made, affect the breaking strain. 



I expect some gentleman present will say what is the use- 

 of all these tests ; it is impossible to use cement in actual 

 practice under such conditions, and it is unnecessary to get a 

 cement giving a breaking strain of 55olbs to join two bricks 

 together, when the bricks themselves are not half as strong.. 

 Now the object to be gained is to find out the very best 

 results that can be obtained from a certain cement under the 

 most favourable conditions. The foregoing are the standard 

 tests, and provided every sample is used in the same way, and 

 under the same conditions, it is easy to get at the comparative 

 values of the different brands. 



It is a mistake to think that because a cement gives a high 

 breaking strain at an early date it is therefore of more value than 

 one which gives a low or moderate strain, for very often it is just 

 the reverse. A cement that gives 65olbs. in 7 days may only 

 give 6oolbs. in 28 days, 55olbs. in three months, and in this 

 way gradually lose strength, until at the end of twelve or eigh- 

 teen months it has all crumbled away, and has no more cohesion 

 in it than sand. It is far safer to have a sample that will 

 show a moderate strain at an early date, and slowly but 

 continually increase in strength ; say 35olbs. in 7 days, 55olbs. 

 in 28 days, 7oolbs. in three months, and so on. High break- 

 ing strains are obtained by the admixture of a heavy 

 percentage of lime ; but it is quite possible to overstep the 

 limit, and if your cement contains more lime than can be 

 neutralised by the silicas, etc., you get free lime, which, 

 while showing great strength to start with, ultimately disin- 

 tegrates and rots the bulk away. 



The test for soundness is an important one ; it is carried 

 out by making a small round disc, termed a " pat," about 

 three inches in diameter and a quarter of an inch thick. Mix 

 it with a minimum of water, work it up well, and put it on a 

 piece of glass. This, like the briquette, must be kept away 

 from the sun, fire, or any hot dry place. At the end of 24 

 hours it is put in water and examined from day to day. If at 

 the end of 7 days it shows no signs of cracks or blowing, ycu 

 can pass it as sound ; if, however, there are cracks, further 

 examination is necessary to ascertain the cause. The 

 chances are that the cement is too new and fresh, and if it is 

 allowed to mature for a few days by spreading a sample out 

 in a thin layer, these cracks will not appear with the next 

 pats. If the cracks do occur again, be very chary about using 

 the bulk. 



Test for fineness. The usual specification is that it shall 

 be ground so fine as to leave no residue on a sieve having 



