Polytechnic Association. . 757 



ing with the water used in the preparation of the mortar a small 

 quantity of sulphate of lime, in the form of either natural gypsum or 

 plaster of Paris (burnt gypsum) ; instead of sulphate of lime, proto- 

 sulphate of iron (green vitriol) may be used, or, better still, sulphuric 

 acid ; for the secret of the process is the effect produced by this acid. 

 However, sufficient acid is contained in gypsum to accomplish the 

 end desired, and the use of gypsum is much the most convenient. 

 The mixture is prepared in the pan of the ordinary mortar 

 mill, lime is added, and after being ground for three or four 

 minutes, the sand, burnt clay or other ingredients are added, and 

 the whole is ground for ten minutes more. Thus ordinary 

 lime is at once converted into a cement-mortar which sets rapidly, and 

 can be used for concrete in brick-laying or in plastering at a cheaper 

 rate than lime prepared in the usual way. Lime thus treated with 

 sulphuric acid or sulphates will take up twice as much sand as when 

 slaked. Tiles joined together by Portland cement, after fourteen 

 days were separated by a weight of fifty-six pounds, while selenitic 

 mortar, containing one part lime and five parts sand, under precisely 

 the same conditions, required a weight of 158 pounds to overcome 

 adhesion. 



Dr. Feuchtwanger — That is quite new; for it has always been 

 taken for granted that lime and gypsum do not agree in mortars. 



Dr. Parmelee and Professor Phin expressed a doubt whether the 

 mortar would be as strong as that made by lime and sand. 



"Mr. T. D. Stetson — In this connection it may be interesting to 

 state how gypsum has been used alone as a mortar. In the sinking 

 of the piers of the Harlem bridge, which were sunk by a modifica- 

 tion of the pneumatic process, having cast iron tubes joined together, 

 pushed down into the mud under a pressure of air sufficient to keep 

 the water out, they reached the hard bed about fifty-four feet below 

 mean low water. Then came the job of filling up with masonry 

 inside. This was done under the care of Erastus W. Smith, whose 

 jobs never attract attention because no accident ever happens to 

 them. In England it has been the practice to build up with masonry. 

 Here the plan has been adopted of filling up with hastily laid 

 material, on which grouting was poured and allowed to set. But Mr. 

 Smith found that ground gypsum had the property of swelling 

 enough to make it bind inside of the tube very much more than 

 ordinary mortar would. In the construction of the Harlem bridge, 

 therefore, he built up with gypsum alone. If gypsum in prepared 

 state, called plaster of Paris, had the cohesive strength of mortar, it 



