jreCnANICS AND USEFUL AKTS. 41 



fragments, which are immersed in a solution of alkaline silicate, 

 containing an alkaline carbonate. Tlie solution which answers 

 best is composed of silicate of potash, containing a sullieient 

 number of equivalents of carbonate of potash to avoid the pre- 

 cipitation of the silica, in the following proportions : 0.880 kilog. 

 (1.94 lbs.) of silicate of potash containing 0.255 kilog. (.56 lb.) 

 of carbonate of potash, in 4.54 litres (a gallon) of water, a 

 solution having a specific gravity of 1,200, but which may vary 

 according to the use for which the cement is intended. As, for 

 example, it can be employed of the strength above indicated in a 

 great many cases where the best quality is required ; and, if an 

 ordiuai-y cement is only necessary, it can be diluted with two 

 parts of water to one of the solution. If a cement be required 

 to harden slowly, sulphate of potash may be added to the car- 

 bonate, so that the indurating action of the silica upon the plaster 

 may thus be varied at pleasure. After having left the plaster 

 steeijed in the solution for twenty-four hours or so, it is taken out 

 and left to drain in a compact mass, in order that the diffusion of 

 the solution through the plaster may take place more effectually ; 

 the cement is then taken back to the furnace, and reheated to 

 150° or 250° C. (302° to 482° Fahr.) to drive off all the water, 

 after which it is ground to powder, and can be colored to any 

 desired hue by mixing with a jjigment. — London Builder, No. 

 1210. 



NEW MORTAR. 



The mortar used by the Romans has, in the course of ages, set 

 so strongly as to be equal in hardness to the stones it was used to 

 cement, and its analysis shows that this is due to the abundant 

 formation of silicate of lime throughout the mass. Modern mor- 

 tar, on the contraiy, usually hardens slowly, cracks while harden- 

 ing, has but little adhesion, and its useful effect is simply as a bed 

 for the i^roper support of the stone or brick ujjon its whole 

 surface, and the consequent distribution of the pressures properly 

 over the sustaining masses. Analysis shows little or no formation 

 of silicates, and the carbonate of the quicklime (for it absorbs 

 carbonic acid itself very slowly) is soluble in the rain to which 

 it is exposed, and rapidly dissolves out. Dr. Artus proposes a 

 method of preparation by which the process of silication is much 

 ftivored ; by which, it is said, a mortar may be jjrepared which 

 becomes as hard as cement, does not crack in setting, and may be 

 used as a hydraulic cement under water. This process is as 

 follows : Take good slacked lime and mix it with the utmost care 

 with finely sifted sand ; mix the sand thus prepared with finely 

 powdered quicklime, and stir the mixtiire thoroughly ; during the 

 process the mass heats, and may then be emjjloyed as mortar. 

 Of course, the mixture must be made just as it is to be used. 

 One part of good slacked lime was mixed with three parts of 

 sand, and to this was added three-fourths of its weight of finely 

 powdered quicklime. The mortar thus made was used in a 

 foundation wall, and in four daj'S had become so hard that a piece 

 of sharp iron would not attack it. In two months it had become 

 as hard as the stones of the wall. 

 4* 



