260 W. S. DE LISLE ROBEKTS. 



independent of the operator, how much water should be used for 

 the kind of cement under test. 



This point was open to so much criticism, that it became 

 absolutely necessary to devise some means of overcoming the 

 difficulty. After various experiments, an apparatus was decided 

 upon, based on the principle that under a given pressure, a given 

 quantity of cement of one quality, will always absorb the same 

 percentage of water. 



The method of operating is as follows : — A weighed quantity of 

 cement is placed in a mould, into which a piston fits, pressure is 

 then applied to the top of the piston by means of a screw, and a 

 spring underneath the mould is depressed to a certain point ; water 

 is then poured in around the mould, then the water is first drawn 



, and afterwards the pressure is released, the cement is then 

 taken out and weighed again, and the difference in weight will 

 give the percentage of water absorbed, which is the percentage 

 that must be used in making the briquettes. By varying the 

 pressure, the amount of water absorbed is as follows for one kind 

 of cement: — (See diagram No. 1.) 



From this it will be understood how great an influence the 

 pressure and percentage of water has over the strength of cement. 



The manner in which cement briquettes are kept until tested, 

 will sometimes vary the strength very much. The usual rule is 

 to keep them in air in a damp place for the first twenty-four hours 

 and the rest of the time in water ; but if allowed to become in the 

 least dry during the first day, the strength is afterwards affected; 

 the same thing happens if the briquettes are taken out of the 

 water any time during the process of hardening, and put back again. 



Diagram 2 shows the the progressive increase in strength of 

 neat cement, from one to three days, under different treatments. 

 No. 1 represents the increase in strength of briquettes placed in 

 water immediately after being made ; No. 2 the same cement kept 

 out of water for twenty-four hours, and the rest of the time in 

 water ; No. 3 the first two days in air and the third day in water. 



