266 W. S. DE LISLE ROBERTS. 



water at 180° Fahr., as twenty-eight days in cold water at 65°. 

 As already shown a certain small percentage of free lime will 

 lower the strength of the neat cement but not always the mortar, 

 on the other hand cement which has a buff surface, and bluish- 

 grey section after drying, will give a tensile strength after seven 

 days in hot water very much higher than twenty-eight days in 

 cold water. A light grey section indicates defective burning, and 

 if unslaked will always show defects in hot water. 



On the table is a briquette two years old which contained a 

 large quantity of slaked lime; the cement gave a high tensile strain 

 at first, with a good progressive increase in strength, but set very 

 slowly ; it remained sound until the second year, when broken 

 after two years in water, the result was an imperfect fracture, 

 showing the formation of CaC0 3 internally. 



The author also showed a specimen of hydraulic lime (silicate 

 of lime) cracked by internal expansion after being exposed to the 

 weather for twelve months, and the same silicate of lime when 

 mixed with sand under the same conditions in weather, but per- 

 fectly sound, the voids being filled with Ca00 3 . 



The relation of cement containing unslaked free lime to the 

 strength after twelve months in water will be understood by the 

 following statistics : Taking the tests made by the Sewerage 

 Department for three years 1891-92-93, the average tensile 

 strength after twelve months of all test, (about two hundred), made 

 during 1891 = 430R>s. per square inch for mortar in the proportion 

 of three to one. And the percentage of tests that proved unsound 

 in Deval's hot bath 3*3%. During 1892 the average tensile strain 

 after twelve months dropped to 372flbs. and the tests unsound in 

 Deval's hot bath rose to 5*1%. During the first half of 1893 the 

 average tensile strain, twelve months is 304Ibs. only, and the tests 

 which disintegrated in Deval's hot bath 6-1%, besides about 10% 

 which were partially disintegrated. This would show that cement 

 as at present imported is very inferior to that imported three 

 years ago ; probably the present price of cement does not make it 

 sufficiently remunerative for the manufacturers to take the proper 

 care required for the manufacture of a sound cement. 



