440 FILLING BAROMETER TUBES. 



frequently. It should then be left for some days, and shaken occa- 

 sionally; then well washed with pure water and dried. I distil first, 

 and then wash with acid, and this will take out the metals likely to be 

 found in it. 



II. — By W. Wurdemann, op Washington. 



In compliance with your request, as contained in your note received 

 this day, I will give some notes in regard to my usual method of filling 

 barometers. 



First, let me premise that I have so far filled only such as have a 

 straight tube, without bend or contraction, and to such alone the 

 method below explained is applicable; nor ought the tubes to be of a 

 less bore than T 3 g- of an inch. 



Besides the requisites stated, those of a clean tube and perfectly 

 pure mercury are equally indispensably necessary with this method 

 as well as any other, where a perfect instrument is desired. The 

 purification of mercury is best accomplished by means of perchloride 

 of iron, with which it is shaken in a diluted state; then carefully 

 washed with pure water, and again freed from moisture by heating. 

 The glass tube must have its open end ground straight and smooth, 

 so that it can be closed air-tight with the finger, or better, with hard 

 caoutchouc, as the former is liable to introduce moisture or grease. 

 Warm well both mercury and glass tube, and fill in through a clean 

 paper funnel with a very small hole (about -g- 1 ^ of an inch) below, to 

 within about one-fourth of an inch of the top. Shut up the end and 

 turn the tube horizontal, when the mercury left will form a bubble 

 that can be made to run from one end to the other by change of in- 

 clination, which will gather all the small air bubbles visible that 

 adhered to the inside of the glass tube during filling. Now let that 

 bubble, which has grown somewhat larger, pass to the open end. 

 Fill up this time with mercury entirely, and shut up tightly. Then 

 reverse tube over a basin, when, by slightly relieving the pressure 

 against the end, the weight of the column of mercury will force some 

 out, forming a vacuum above, which ought not to exceed one-half an 

 inch. Closing up again tightly, let this vacuum bubble traverse the 

 length of the tube on the several sides, when it will absorb those 

 minute portions of air, now greatly expanded from removed atmo- 

 spheric pressure, that were not drawn at the first gathering. 



The perfect freedom from air is easily recognized by the sharp con- 

 cussion with which the column beats against the sealed end, when, 

 with a large vacuum bubble, the horizontally held tube is slightly 

 moved. 



