28 Verification of Thermometers at Freezing-point of Mercury. 



heavy iron bottles so familiar in lecture-rooms as recipients of 

 compressed oxygen. A convenient quantity for the prosecu- 

 tion of an experiment is 200 gallons of gas, which weighs 

 about 3 lb. The quantity of mercury operated upon is from 

 17 to 20 lb. By means of a common salt and ice freezing- 

 mixture, the vessels, thermometers, mercury, and ether are all 

 cooled down to about —10° C. beforehand. 



The collecting-box being applied to the nozzle of the gas- 

 receiver, the valve is unscrewed and gas allowed to escape 

 through the apertures in the handles of the box for about a 

 minute, during which time about 550 grains of solid carbonic 

 gas are deposited in the form of snow. 



The box being opened, the snow is removed with an ivory 

 paper-knife and laid on the surface of the mercury previously 

 placed in a wooden box well jacketed with felt. 2 oz. of ether 

 is then poured over it, and the pasty mass forced under the 

 surface of the mercury by means of a special wooden stirrer. 

 The liberated gas and ether-vapour escaping through the 

 mercury rapidly cool its temperature down, and a thin film 

 of solid mercury forms on the top of the mass, which is 

 immediately broken up by the stirrer. About six charges of 

 acid snow and six fluid ounces of ether are usually necessary 

 to convert the whole of the mercury into the finely granular 

 mass of a pasty consistence. 



It is considered inadvisable to freeze the mercury into a 

 solid state, as then it becomes necessary to take it out of the 

 box, and cut it up with shears into small lumps before the 

 thermometers can be inserted into it, an operation which 

 entails great loss of time and frozen mercury. 



When the mass is in the semi-solidified condition the ther- 

 mometers are plunged into it and allowed to fall to the 

 requisite temperature — a well-verified standard being always 

 placed in the bath at the same time and carefully watched 

 during the progress of the comparison. 



As soon as this standard is seen to begin to rise above the 

 lowest point, the thermometers are taken out, and an additional 

 quantity of snow and ether forced into the mercury until its 

 temperature is again reduced, and the feel of the thermometer 

 shows it has regained the proper consistency. 



When the operation is properly and carefully performed, it 

 is found that the work of comparison may be spread over 

 from half an hour to forty minutes, a period which permits of 

 more than 100 mercurial thermometers being read with the 

 necessary exactitude. If the thermometers are ordinary 

 alcohol or Rutherford minimums with large bulbs, only from 

 thirty to forty can be compared at a time. 



