508 



RUDBERG ON THE EXPANSION OF DRY AIR. 



taining from 120 to 150 grammes of mer- 

 cury, was used for containing the air. After 

 the end of the tube had been fitted into a 

 hole in a cork at one end of a cyhnder, D E, 

 containing chloride of calcium, the air was 

 dried, either by heating the globe strongly over 

 a spirit-lamp, and then suffering it to cool, 



and repeating the process at least fifty or sixty ^ 



times ; or else by connecting the end E of the 

 cylinder with an air-pump, and exhausting and 

 re-admitting the air fifty or sixty times. I have 

 not observed any difference between these two 

 methods of diying air, but have found one as 

 effectual as the other. The chloride of cal- 

 cium was fused at a red heat, then poured out 

 upon a cold plate of metal, and as soon as it 

 became solid, broken to pieces, and put into 

 bottles with ground stoppers while red hot. 



The globe having been dried in this manner, and remaining 

 in connexion with the chloride of calcium tube, a small opening 

 being made in the cork at E for the air to escape through, was 

 suspended by means of a cork G cut in two, in the boiler F, the 

 upper part of which, as described in my memoir on the construc- 

 tion of thermometers (Poggendorfi's Annalen, B. 40.), consists 

 of two concentric cylinders, so that the globe and the greater 

 part of the tube were surrounded by steam. After the water 

 had been boiling three quarters of an hour, or an hour, the cy- 

 linder D E was removed, and the boiUng continued for about 

 ten minutes longer. The height of the mercury in the barome- 

 ter was then observed, and the tube sealed, the water being kept 

 boiling freely in the mean while. 



After the ball had been weighed with a balance, which turned 

 with one-tenth of a milligramme, it was firmly fixed to the arm 

 Q, (fig. 2.) of a steady support, with the tube passing through 

 a hole, in a metal dish H. The arm Q was then so far de- 

 pressed, that the point of the tube was deeply immersed in 

 the mercury cf the trough T. Lastly, the point of the 

 tube was broken off, and in order that all the mercury re- 

 quisite might enter, the ball was suffered to remain in this 

 situation several hours, almost always all night, although I 

 had convinced myself that not more than a quarter of an hour 



