1893.] 



On the Densities of the Principal Gases. 



187 



surfaces, is observed with the aid of lenses of about 30 mm. focus, 

 held in position npon the wooden framework of the apparatus. It 

 is, of course, independent of any irregular refraction which the tube 

 may exercise. The verticality of the line joining the points is tested 

 without difficulty by a plumb-line. 



The upper and lower chambers C, B are formed from tubing of the 

 same diameter (about 21 mm. internal). The upper communicates 

 through a tap, D, with the Toppler, by means of which a suitable 

 vacuum can at any time be established and tested. In ordinary use, 

 D stands permanently open, but its introduction was found useful in 

 the preliminary arrangements and in testing for leaks. The con- 

 nexion between the lower chamber B and the vessel in which the 

 pressure is to be verified takes place through a side tube, E. 



The greater part of the column of mercury to which the pressure 

 is due is contained in the connecting tube FF, of about 3 mm. in- 

 ternal diameter. The temperature is taken by a thermometer whose 

 bulb is situated near the middle of FF. Towards the close of 

 operations the more sensitive parts are protected by a packing of tow 

 or cotton-wool, held in position between two wooden boards. The 

 anterior board is provided with a suitable glass window, through 

 which the thermometer may be read. 



It is an essential requirement of a manometer on the present plan 

 that the measuring rod pass air-tight from the upper and lower 

 chambers into the atmosphere. To effect this the glass tubing is 

 drawn out until its internal diameter is not much greater than that 

 of the rod. The joints are then made by short lengths of thick- walled 

 india-rubber H,G, wired on and drowned externally in mercury. The 

 vessels for holding the mercury are shown at I, K. There is usually 

 no difficulty at all in making perfectly tight joints between glass 

 tubes in this manner ; but in the present case some trouble was exper- 

 ienced in consequence apparently of imperfect approximation between 

 the iron and the mercury. At one time it was found necessary to 

 supplement the mercury with vaseline. When tightness is once 

 obtained, there seems to be no tendency to deterioration, and the con- 

 dition of things is under constant observation by means of the 

 Toppler. 



The distance between the points of the rod is determined under 

 microscopes by comparison with a standard scale, before the appa- 

 ratus is put together. As the rod is held only by the rubber con- 

 nexions, there is no fear of its length being altered by stress. 



The adjustment of the mercury (distilled in a vacuum) to the right 

 level is effected by means of the tube of black rubber LM, ter- 

 minating in the reservoir N. When the supply of mercury to the 

 manometer is a little short of what is needed, the connexion with 

 the reservoir is cut off by a pinch-cock at 0, and the fine adjustment 



