Butler and Sheridan — Account of a New Oedanometer. 473 



To fit up tlie apparatus, the rubber diaphragm and washer are set in place, 

 and the two eastings bolted togetlier, care being taken to ensure an even 

 seating by tightening the screws each half a turn in rotation. Witli the 

 needle-valve closed, the upper casting E is filled with clean mercury, care 

 being taken to expel all air. The valve is then opened to displace the air 

 around tlie needle. The air in the manometer tube may be dried rapidly by 

 connecting the end of the tube, through a wide glass tube containing calcium 

 chloride, with a vacuum pump, and exhausting the air several times. Again, 

 with the aid of a vacuum pump, a thread of mercury is allowed to enter up to 

 tlie graduated part of the manometer tubing, wliich is then dipped into the 

 mercury in the upper part of the casting. The packing rubber D, the 

 washer C, and gland B are adjusted ; surplus mercury escapes through tube S, 

 by means of a flexible tube and funnel attached to it; the lever of the 

 mercury in the manometer is read oft' at atmospheric pressure. Tlie needle- 

 valve is then tightly closed, and the tube S covered by a cap and the apparatus 

 inverted. The seed is placed on the rubber diaphragm and carefully packed 

 round with sand, the disc M is inserted, and water poured in to remove the 

 air and wet the sand. 



The fine sand used was sieved, so as to contain only grains of approxi- 

 mately uniform size. It was treated with nitric acid, and washed to remove 

 impurities. The plug K, tlie thread of which is greased to make it water- 

 tight, is screwed home with considerable pressure to cause the firm packing of 

 the sand. This causes the mercury to rise in the manometer tube ; tlie 

 needle-valve is subsequently opened to adjust the pressure. It was found 

 advisable to allow a small initial pressure so as to diminish the amount of 

 swelling permitted to the seed. If desired an initial pressure of over 

 50 atmospheres can be obtained by placing five or six steel balls, similar to 

 those used in motor-bicycle bearings, between the disc and the plug K, thus 

 diminishing friction. The tubes L are now connected with the water-supply 

 system. The volume of the graduated portion of the bore of our manometers, 

 of whicli we had six constructed, varies somewhat, but has an average of about 

 100 cms. 



The pressure is calculated from the volume of the gas, according to 

 Boyle's law, and a correction is made for the height of the column of mercury 

 measured from the diaphragm. 



Between temperatures of lO'^ and 100° C. the aberration from Boyle's law, 

 due to the volume occupied by the molecules and the mixed chemical nature 

 of air, &c., for pressures up to 50 atmospheres is negligible, so that a correction 

 on this account is unnecessary. 



We have estimated that the bean swelling freelj' increases on an average 



