18 DENSITIES OF OXYGEN AND HYDROGEN 



\2. — MEASUKEMKNT OF THE DEGREE OF EXHAUSTION EFFECTED. 



Tlie ]>erfection of the vnc'iiimi iirodiicecl in each exjierirnent was measured with 

 tlie i,MUi^e devised by McLeod. The cai)ac"ity of tlie bull) of that one which was 

 set up with the Toepler pumj) was about HOo cubic centimetres. On the tube 

 were three marks, such that one millimetre of difference of level in the two tubes 

 showed a vacuum of one millionth or five millionths of an atmosjdiere, or one 

 measured l»y the two-hundreth of a millimetre of mercury in a syphon gauge. 



The gauge used with the (xeissler pump had a Imll) whose capacity was 436 

 cubic centimetres. On the tul>e were six marks; the upper, third, and fifth indi- 

 cate vacua of the ten-millionth, the millionth, or the hundred-thousandth of an 

 atmosphere, by a diffeience of level of one millimetre; the second, fourth, and 

 lower marks indicate in the same way, vacua expressed by one ten-thousandth, one 

 thousandth, or one hundredtli of a millimetre of mercury in a syphon gauge. 



The tubes of the gauge belonging to the Toepler pump were of such size, and 

 kept so clean, that difference of capillary action did not inucii interfere with the 

 accuracy of its indications. This was often tested by exhausting the gauge to a ten- 

 millionth of an atmosphere, when any irregularity in the level of the mercury in 

 the j>art of the tube where the readings were connnonly made could lie detected. 

 The eri'or found has never been important In comparison with the (|nantities to be 

 nieasui'ed. 



l.'t. MANIPULATION OF til-OHE WHILE KIM.INii Wnil oXVOEN. 



In the present series of experiments, an elaborate apparatus was used to 

 measure the amount of inipuiity in the o.xygen. The method recpiired that while a 

 globe was filling with oxygen or hytlrogen to be weighed, a second globe should 

 also be filled with a quantity of the gas absolutely identical in quality for sub- 

 sequent analysi-s, and that this sam[)le should be absolutely safe from leakage. 

 Now, when measurements were made in this way of the amount of impurity in the 

 oxygen used for weighing, it was found to be negligible. But some experiments 

 which proved this in another way are easier to describe, and are conclusive ; so that 

 perhaps it is not woith while to detail more at length the apparatus really used at 

 the time the present series of determinations were made. 1 therefore describe only 

 the manipulation retpiired in filling the glolx-. 



The globe </., Fig. tl, which had been previously exhauste<l, was placed in the 

 case 1)1 111, in which^'/'is a non-conducting envelope, and dd is a nniss of watei- con- 

 tained in a vessel consisting of two concentric cylinders of iron. Atc^eis a stopper 

 for the mouth of the inner iron cylinder: it consists of a cylinder filled with water. 

 In this stopper are three tubulatures f<ir the two thermometers and for the tube 



