28 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



PURIFICATION OF THE MERCURY. 



The material which ordinarily passes for pure mercury in the labora- 

 tory is by no means suitable for manometric work, and to obtain it 

 in adequately pure condition for this purpose requires unusually 

 thorough treatment. The mercury which is used in our manometers — 

 and also that which is now used in the bath thermostats — is cleansed 

 in the following manner: 



1. The commercial material is first filtered through paper filled with 

 pin holes to free it from dirt. It is then heated for four hours to the 

 boiling-point in a glass retort, to the neck of which a long glass tube 

 has been fused for the condensation and return of the vapors; and 

 during this time a current of air is forced through the boiling metal. 

 On cooling, it is again filtered to remove the scum of oxides which 

 usually forms in considerable quantity. 



2. It is distilled in a vacuum. 



3. The distillate is washed by the method of Lothar Meyer, but 

 with water containing 2 per cent of nitric acid and 2 per cent of mer- 

 curous nitrate instead of ferric chloride. The apparatus in which the 

 washing is done consists of a wide tube two meters in length, to the 

 lower end of which has been fused a quite narrow tube of the usual 

 double U form, the proportions of the descending and ascending limbs 

 being so selected that the mercury which supports the cleansing liquid 

 shall lie wholly within the smaller tube. To admit the mercury at 

 the top and to regulate its flow, a separating funnel is employed. The 

 lower end of the funnel, instead of being drawn out to a fine point, as 

 in the apparatus of Meyer, is widened out into the form of an inverted 

 funnel, according to the suggestion of Hillebrand, and over this are tied 

 two or three thicknesses of the finest silk bolting-cloth. The material 

 to be purified is thus made to enter the cleansing liquid in hundreds and 

 perhaps thousands of excessively fine streams. It is passed 1,000 times 

 through the solution of nitric acid and mercurous nitrate, and is then 

 thoroughly washed with water and dried. 



4. After treating the mercury as described under 1, 2, and 3, it is 

 again distilled in a vacuum, but not in the still (2) which is used for 

 the first distillation. 



The mercury which has thus been cleansed retains its brilliant luster 

 in the air, and its movements in narrow tubes are highly satisfactory. 

 We have also prepared mercury from the purest oxide which we could 

 make, but have not found it superior in any way to the product 

 obtained by the means desribed above. 



CALIBRATION OF THE MANOMETERS. 



The tubes which are used in making the manometers are the most 

 nearly perfect for the purpose which it is practicable to obtain. The 

 essential requirements are that any tube shall be of very nearly uni- 



