1893.] 



On the Densities of the Principal Gases. 



135 



gases, unsystematic errors of weighing are less to be feared than 

 doubts as to the actual temperature. 



In order to secure the unsystematic character of these errors, it 

 is necessary to wash and wipe the working globe after an exhaustion 

 in the same manner as after a filling. The dummy globe (of equal 

 external volume, as required in Regnault's method of weighing gases) 

 need not be wiped merely to secure symmetry, but it was thought 

 desirable to do so before each weighing. In this way there would 

 be no tendency to a progressive change. In wiping the globes the 

 utmost care is required to avoid removing any loosely attached grease 

 in the neighbourhood of the tap. The results to be given later will 

 show that, whether the working globe be full or empty, the relative 

 weights of the two globes can usually be recovered to an accuracy of 

 about 0*3 milligramme. As in the former papers, the results were 

 usually calculated by comparison of each "full" weight with the 

 mean of the immediately preceding and following empty weights. 

 The balance and the arrangements for weighing remained as already 

 described. 



The Manometer. 



The arrangements adopted for the measurement of pressure must 

 be described in some detail, as they offer several points of novelty. 

 The apparatus actually used would, indeed, be more accurately spoken 

 of as a manometric gauge, but it would be easy so to modify it as to fit 

 it for measurements extending over a small range. 



The object in view was to avoid certain defects to which ordinary 

 barometers are liable, when applied to absolute measurements. Of 

 .these three especially may be formulated : — 



a. It is difficult to be sure that the vacuum at the top of the 



mercury is suitable for the purpose. 



b. No measurements of a length can be regarded as satisfactory in 



which different methods of reading are used for the two 

 extremities. 



c. There is necessarily some uncertainty due to irregular refraction 



by the walls of the tube. The apparent level of the mercury 

 may deviate from the real position. 



d. To the above may be added that the accurate observation of the 



barometer, as used by Regnault and most of his successors, 

 requires the use of a cathetometer, an expensive and not 

 always satisfactory instrument. 



The guiding idea of the present apparatus is the actual applica- 

 tion of a measuring rod to the upper and lower mercury surfaces, 

 arranged so as to be vertically superposed. The rod AA, 6g. 1, is of 

 iron (7 mm. in diameter), pointed below B. At the upper end, C, it 



