442 CONSTRUCTION OF A STANDARD BAROMETER. 



larger tube, was finally bent into a syphon. The end of the lower 

 capillary tube was now connected with a good air pump, and the air 

 very slowly extracted at the same time that the whole tube was 

 strongly heated by passing a large spirit lamp along it. "When the 

 air had been as well as possible extracted, and whilst the air pump 

 was still in action and the heat still applied, the lower capillary tube 

 was sealed by a blow-pipe flame. When the tube had cooled, it was 

 placed at a small inclination with the end of the upper capillary tube 

 in a vessel containing mercury which had been previously boiled. 

 The point of this tube was broken off under the mercury, which then 

 rose in the tube by atmospheric pressure. The mercury continued 

 to rise until the bulb at the other end was more than half filled, the 

 remaining space being occupied by the air which the pump had failed 

 to extract. It was estimated from the amount of space thus left unoc- 

 cupied by the mercury that the pressure of the residual air in the tube 

 when cold must have been less than five hundredths of an inch. The 

 basin of mercury was then withdrawn from beneath, leaving the point 

 of the capillary tube exposed, the bore of which remaining quite filled 

 with mercury. The blow-pipe was then applied to the point, and the 

 opening sealed. When the glass had cooled, the large tube was placed 

 erect, the mercury separating at the contracted part of the capillary 

 tube, leaving the remainder filled, or very nearly so, and the part 

 between the point of contraction and the large tube a vacuum. The 

 upper capillary tube was now sealed at about the middle of the vacuum, 

 and the remaining portion removed. Finally the syphon tube at the 

 lower end of the large tube was broken under mercury, leaving about 

 an inch of the syphon remaining. 



The earlier tubes filled by this process were not satisfactory, there 

 being, as in those previously prepared by boiling, a considerable adhe- 

 sion of the mercury to the glass, with the formation, after a few days, 

 of rings of dirt; so similar, indeed, was the appearance of these tubes 

 to that of the boiled tubes, that I was led to believe that the evil in 

 both cases was due to the same cause. Being satisfied that there was 

 no impurity in the mercury, which, besides having been cleaned with 

 nitric acid, had before these last experiments been redistilled, and 

 suspecting that the evil might have been owing to imperfect cleaning 

 of the tubes, which had only been ivi-ped out by the glassblower in the 

 usual way, I had fresh tubes made under my own inspection, and sealed 

 at the glass-works immediately after being drawn. Great care was 

 also taken by the glass-blower to prevent the entrance of moisture 

 during the subsequent operations with the blow-pipe. These tubes, 

 however, still showed the same imperfection, though in a less degree. 

 About this time I had the advantage of consulting Mr. John Adie, of 

 Edinburg, who informed me that he had also experienced the same 

 inconvenience, and that he had removed it by thoroughly cleaning 

 the tubes by sponging with whiting and spirits of wine. Following 

 his directions, 1 had the satisfaction of finding the tubes when filled 

 almost wholly free from the imperfections mentioned. A tube of 

 1.1 inch internal diameter, prepared in July, 1855, by the process 

 above described, is at this time in as good condition as when first 



