594 Transactions of the American Institute. 



grain of strontia is easily detected, and has been shown to exist in very 

 many limestones of various geological ages. 



4. Lime.— 6-100,000th of a milligramme or l-l,000,000tli of a 

 grain of lime can be easily detected. 



• 5. CcBsium. — This new alkaline metal was discovered by Bnnsen, 

 in the mineral waters of Baden, Durkheim. Its spectrum consists 

 of two bright blue lines. Thirty tons of mineral water yield 100 

 grains of caesium. 



VIII. The Strength of Portland Cement. 

 Mr. John Grant, in a paper recently read before the Institution of 

 Civil Engineers, London, described a series of experiments made by 

 him for the purpose of testing the strength of Portland cement, also 

 the different modes of using cement. A sewer constructed of con- 

 crete, consisting of one-seventh cement to six-sevenths of sand, and 

 lined inside with cement, was regarded by him as the cheapest form 

 of sewer, combining strength with soundness. Tables were also given 

 of the strength of 589,271 bushels of Portland cement used during 

 the last five years on various works south of the river Thames, show- 

 ing an average tensile strain at the end of a week of 806.63 pounds, 

 equal to 358.5 pounds per square inch, being an improvement on that 

 reported five years ago of eighty-nine pounds per square inch. At 

 the end of thirty days, 37,200 bushels of the same cement, ascertained 

 by 1,180 tests, had an average strength of 455 pounds per square inch. 

 Further experience had confirmed the earlier conclusions that the 

 strength of Portland cement increased with its specific gravity, its 

 more perfect pulverization, and its thorough admixture with the mini- 

 mum quantity of water in forming mortar. Heavy cement, weighing 

 123 pounds per bushel, took about two years to attain its maximum 

 strength when used pure ; but by the admixture of sand or gravel, 

 cement, mortar, or concrete was reduced in strength, and set less rapidly 

 than pure cement. Koman cement, though i;-om its quick setting 

 property very valuable for many purposes, deteriorated after exposure 

 to air before use, about twice as much as Portland cement, if meas- 

 ured by strength. In making cement concrete, it would, from this, 

 seem desirable to spend no more time than was absolutely necessary 

 to effect a thorough admixture of the cement with the sand and 

 gravel. 



Compensating Car-wheel. 

 Mr. W. E. Partridge exhibited and explained a model of W. F. 

 Frayer's compensating railroad car-wheel. The wheel is constructed 



