37 



tallow, or, if available, special stopcock lubricant, very sparingly 

 rubbed over the external surfaces of the tubes. A little pure mercury 

 is next filtered into the bend, or U-shaped section. Small air bubbles, 

 if any appear, should be excluded by tilting the tube and causing the 

 mercury to flow about in a manner that will accomplish this result. 

 When the mercury covers the ground surfaces the short branch of 

 the siphon should be carefully inserted - and the whole secured to the 

 instrument in the manner provided. More pure ' mercury is now 

 added to the open cup until it is filled nearly to the brim. As some 

 mercury is likely to be spilled in the course of subsequent operations, 

 it is a good plan to have a clean porcelain or glass photographer's 

 tray close underneath the plate supporting the bend. This will serve 

 to catch any mercury that may escape. 



75. The long branch of the siphon, completely filled with clean 

 mercury, is now lifted, and, while the open end is temporarily closed 

 firmly with the finger tip, the tube is carefully inclined in a manner 

 that will permit the finger and point to be dipped below the free 

 surface of mercury in the cup. Still supporting the weight of the 

 heavy tube so that the submerged end does not bear with undue 

 pressure upon the parts of the cup, the whole is carefully and slowly 

 brought into a vertical position. When the elevation of the tube 

 has reached the point at which the mercury begins to leave the top 

 of the tube, an assistant should be ready to catch in a suitable vessel 

 (a dry, clean, drinking glass will answer very well) the excess of 

 mercury that overflows from the open cup. 



The heavy tube must be fully supported until quite vertical, and 

 the end only then inserted into the ground joint and rotated a little 

 as it is faced to the front. 



76. Certain precautions must be observed throughout the opera- 

 tions we have just described. (1) The tip end of the tube must not, 

 under any circumstances, be lifted out of the mercury after the finger 

 is removed. (2) After the flow of mercury has started the elevation 

 of the tube must be made gradually; otherwise the column of mer- 

 cury will tend to oscillate or surge up and down and may uncover 

 the point of the tube in the cup. (3) Any lowering of the tube causes 

 the mercury to recede into the vacuum, and will empty the cup unless 

 the supply is kept up by pouring back some of the excess that has 

 already overflowed. 



77. Having finally seated the long branch, some of the excess of 

 mercury must be restored to the siphon and the level brought up to 

 the proper point in the open leg. At the completion of these opera- 

 tions one of the cups of the bend is full to overflowing with mercury, 

 and the other is nearly or quite empty. Some of the mercury in the 

 full cup can easly be removed by splashing it out into a cup held to 

 receive it, using a piece of card or ivory paper folder for the pur- 

 pose. A little mercury may be added to the empty cup. 



78. To clean the mercury. — ^When the glass and mercury in the 

 open leg become soiled through prolonged use, all that is necessary 

 after removing the float is to loosen carefully the short branch of the 

 siphon and permit the excess of mercury to overflow into a clean 



^ The alignment of the ground joints of the siphon can never be made quite perfect, 

 and the 2 branches are marked with a side that when faced to the front gives the best 

 results. In seating the tubes they should be faced in this position. 



