264 W. S. DE LISLE ROBERTS. 



continues in every case at the same ratio as the average increase 

 for that class of cement minus the strength lost by the break- 

 down. 



The partial break-down which becomes more serious, the richer 

 the cement, has a great effect upon the cement facing of buildings 

 — the facing is generally finished with a coating in the proportion 

 of two to one or even one to one ; and there is very little if any 

 cement manufactured which could remain so constant in volume, 

 when exposed to the weather as not to show cracks after a few 

 months near the surface. 



Now if the present practice were reversed, viz., the richer 

 rendering put on first, and a finishing coat of three or even four 

 to one — or better still a proportion of three or four to one all 

 through — there would be sufficient voids to allow of any expansion, 

 and the formation of carbonate of lime would by gradually filling 

 up the voids, add so greatly to the strength, that it would be in 

 some cases as strong or stronger than the neat cement after a few 

 months, and remain perfectly free from cracks. 



As no cement is ever made chemically perfect, there is always 

 a certain amount of free lime, or feebly combined lime, which 

 generally makes itself apparent when the cement is so rich that 

 there are no voids left. 



Now it might be urged that no cement should be used with 

 over a certain proportion of free lime ; that is very true, but there 

 is a certain amount of decomposition of the more feeble compounds 

 with lime, especially when exposed to the sun and rain, thus sett- 

 ing more lime free which then becomes (by contact with C0 2 in 

 moist air,) carbonate of lime. In diagram No. 8, is shown the 

 percentage increase of carbonate of lime in samples kept under 

 different circumstances, unhydrated, and kept in air ; hydrated 

 and kept in air ; hydrated and kept in water for a period of twelve 

 months ; with the dry cement there is hardly any change, show- 

 ing that cement may be kept in a good condition for that length 

 of time. The hydrated cement kept under water decomposes very 



