508 
MR, WELSH ON THE CONSTRUCTION OF A STANDARD 
a larg-e spirit flame along- it. When the air had been as well as possible extracted, and 
whilst the pump was still in action, and the heat still applied, the capillary tube, GH, 
was sealed at I by a blowpipe flame. When the tube had cooled, it was placed at a 
small inclination with the end F in a vessel containing mercury which had been pre- 
viously boiled. The point F was broken off under the mercury, which then rose in 
the tube by atmospheric pressure. The mercury continued to rise until the bulb at 
G 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 
unoccupied by the mercury, that the pressure of the residual air in the tube, when 
cold, must have been less than 0-05 inch. The basin of mercury was then with- 
drawn from beneath the tube, leaving the point F exposed, the small tube remain- 
ing quite filled with mercury. The blowpipe was then applied at F and the opening 
sealed. When the glass at F had cooled the tube was placed erect, the mercury 
separating at the contracted part D, leaving the tube from D to F filled, or very 
nearly so, and from D to A empty. The tube was now sealed at K, the portion 
KDEF being removed. Finally, the lower end of the tube was broken under mer- 
cury at C, leaving about an inch of the syphon. 
The earlier tubes filled by this process were not satisfactory ; there being, as in 
those previously prepared by boiling, a considerable adhesion 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 1 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 wiped out 
by the glassblower in the usual way ; I had fresh tubes made under my own inspec- 
tion and sealed at the glass-works immediately after being drawn. Greater care was 
also taken by the glassblower to prevent the entrance of moisture during the sub- 
sequent operations witli the blowpipe. These tulies, however, still showed the same 
imperfection, though in a less degree. About this time I had the advantage of con- 
sulting Mr. John Auie of Edinburgh, 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, I had 
the satisfaction of finding the tubes when filled almost wholly free from the imper- 
fections mentioned. A tube of IT inch internal diameter, prepared in July 1855 by 
the process above described, is at this time in as good condition as when first erected. 
The top of the column presents a good convexity in all states of the barometer, with 
only a very slight trace of dirt. No appearance of air-specks can be detected, except 
a few very minute ones near the lower end of the tube, whieh have existed since the 
commencement, and were produced by the temporary entanglement of a small air- 
bubble at the shoulder L of the tube in the operation of filling. These specks have 
