60 



The vessel may be just a little warm with advantage. Filter into the 

 vessel, through a paper funnel, such as described below, rather less 

 than a teaspoonful of pure mercury. If the mercury has been prop- 

 erly filtered and is of extreme purity, the brilliant globule will roll 

 about the cup with the greatest activity, as the latter is moved a 

 little, and will draw out momentarily into slender cylindrical por- 

 tions which, if broken asunder, will quickly separate into smaller 

 portions which draw themselves up into beautiful little spheres or 

 larger rounded buttons, none of which clings in the slightest degree 

 to the clear surface of the vessel. Under favorable conditions and 

 during the rapid movements of the mercury a scarcely audible but 

 still a very characteristic crackling sotmd can be heard, due to the 

 development of small sparks of electricity. Such is the character- 

 istic behavior of clean, pure mercury in a clean porcelain dish. If, 

 however, the mercury contains the most minute trace of lead, tin, 

 zinc, etc., this fact will be shown by a more or less marked tendency of 

 the little, slender portions of the mercury to draw out into sharply 

 pointed, tapering "tails," the tip ends of which cling to the vessel 

 and remain. If the observer is not in possession of the small quan- 

 tity of extremely pure mercury needed in the above experiment, the 

 most striking part of it will be lost. After watching the beautiful 

 manner in which the pure mercury rolls about the dish, add to it a 

 small flake of lead or solder. The flake should be a very small 

 shaving cut off with a penknife, and should contain not nearly so 

 much material as in the head of the smallest pin. Place this upon 

 the mercury and allow it to remain a momenti. It will presently be 

 wholly dissolved. Now repeat the rolling about of the mercury in 

 the cup and observe the wonderful change. 



The former brilliant globule has now a dull surface, with its edges 

 clinging at many points to the surface of the dish. The clean, white 

 surface of the dish will now be soiled and discolored when the mer- 

 cury is made to flow over it a few times. The presence of one part 

 of lead in one hundred thousand parts of mercury is readily shown 

 by this test. 



Only one who has performed this experiment is prepared to appre- 

 ciate fully the importance of absolute cleanliness in barometer cis- 

 terns and the necessity for the avoidance of the slightest metallic 

 contamination of the mercury. 



138. Of course, the mercury used in the above experiment can not 

 be again used until purified. This can be done quite well by wash- 

 ing with dilute nitric acid, about one volume of acid in fifteen 

 volumes of water. The mercury and acid may be placed in a bottle 

 and violently shaken, or the acid may be poured over the mercury 

 and allowed to remain several hours. When the acid has thoroughly 

 cleansed the mercury, the latter, upon the contents of their bottle 

 being violently shaken, will break up into very fine globules which, 

 for a moment, do not coalesce. This formation of the mercury 

 into minute globules in the presense of dilute acid will take place 

 only with quite pure mercury. 



139. Returning now to the process of restoring the barometer, 

 the next step is to filter the mercury and fill up the cistern. Roll up 

 a small sheet of clean paper into a sharp cone, looking through it to 

 the light to see that the opening is very small. Holding the cone 

 over a clean vessel, partly fill it with mercury, By twisting the folds 



