FEW YORK STATE MUSEUM 



water level ifl a rack on which the cement can he placed. The 

 pat ia made and then put on the rack for G to 8 hours, after 

 which it is put ia warm inter from 10 to 16 hours. If at the 

 end ol that time it is firm and adheres to the glees, it can be con- 

 it doefl disintegrate, it may .-imply indicate that 

 the commit is too fresh. Cement is said to blow very often if 



ited - \ honra after making. 



In making tl, it is necessary, specially in the case of slow 



Betting cements, to protect them from the sunlight and drafts. 

 F«'r this reason they are covered with a moist cloth. 



v claims 1 that, according to German experience, all tests 

 to determine blowing with the exception of the German cake 

 method are misleading, and that a swelling of cements (Portland) 



really a rarity. 



>f changeable volume, he maintains, differ in other 

 properties specially their tensile strength, from Portland cement, 

 so that they are easily recognized. Some cements, however, such 

 as highly magnesium ones, will, when burnt to a clinkered con- 

 dition like Portland, refuse to swell when first mixed, and some- 

 times do not show an increase in volume wdien kept under water 

 till nearly a year later, but they then show the property to a 

 marked degree. 



The apparatus used by the Germans for determining change 

 in volume is known as Lauschinger's caliper apparatus, and can 

 be made to show the change in volume that takes place in a speci- 

 men over an ext.-iided ]>eriod of time (pi. 2G). 



It enables one to determine by direct measurement the changes 

 in length of small parallclopipeds of about 100 mm (4 inches) 

 long and i> square cm (.78 square inch) area with an accuracy 

 °f ¥uT7 mm (Wittf °f an inch). The apparatus consists princi- 

 pally of a stirrup-shaped caliper, having a fine micrometer -crew 

 on it.s right arm, the left being the support of a sensitive lever. 

 The shorter arm of the lever terminates in a blunt caliper point, 

 and i- preyed against the measuring screw by a spring attached 

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1 Trine.. Am. fl . iv. OUT. 30: 15. 



