302 G. A. Perkins — Yapor-tension of Sulphuric Acid. 



and consisted in proving that the tension does not change 

 much between 20° and 100° C. : the column of acid, by which 

 the change of tension was measured, being 3 mm in diameter 

 and terminated at one end by a vacuum and at the other by a 

 bulb of gas which was kept as nearly as possible at the same 

 temperature while the acid at the vacuum end of the tube was 

 heated.* 



My own plan originally was to take a U-tube, half filled with 

 mercury, and after drying and exhausting the air from both 

 legs, to observe the depression of mercury in one leg, when a 

 drop of acid was introduced. In every case an elevation was 

 observed, due to moisture leaking in through the cocks, or 

 remaining on the glass and evaporating as exhaustion ad- 

 vanced ; a form was therefore adopted which was free from 

 cocks and in which the mercury could be 

 boiled to drive off all moisture. It consists 

 of two barometers. One is an ordinary cis- 

 tern barometer (I), the other is an inverted 

 U-tube with mercury in both legs. One end 

 (h) dips into the same vessel as the tube I, 

 the other (a) is recurved to prevent the ad- 

 mission of air, but allowing the acid to be 

 introduced by a curved pipette or medicine 

 dropper. The height of the column (h) in U 

 is compared with that in I, before and after 

 admitting acid into U through a. The inter- 

 nal diameter of the tubes is about 0\8 cm . In 

 filling the U-tube, it is exhausted by a Sprengel 

 pump and heated strongly. When cool, a 

 little mercury is introduced and boiled and this operation is 

 repeated till the tube is full. Unless this precaution is taken, 

 the acid in rising through the tube, carries up air, which causes 

 a depression of the mercury. 



The acid was purchased of Queen & Co. as pure concentrated 

 acid, but not absolutely anhydrous. 



In making the measurements from three to five readings 

 were made of the position of the meniscus in U and the same 

 number for I : then acid was introduced into the other leg of 

 U and the readings were repeated in the same order. The 

 mercury column is illuminated from behind and a ring of 

 black paper, sliding on the tube, is adjusted as near as possible 

 to the top of the meniscus, allowing only a thread of light to 

 pass over the top of the column. Unless this is done, reflec- 

 tions will make the top of the column uncertain. Where it is 

 possible, the method of using the image of a black point and 



* Encyc. Brit., 9th ed., Article "Heat." 



