118 
DR. M. IV TRAVERS, MR. G. SEXTER, AXD DR. A. JAQUEROD 
which for this purpose was connected through a l)ulb x and a tube 2 to the apparatus 
containing the gas, and to a mercury pump. The tube x was intended to receive 
the mercury contained in the portion of the side tube above the stop-cock. 
By lowering the reservoir connected by the rubber tube / with the stop-cock tXy 
tlie level of the mercury in li could be made to fall below the point of entry of the 
side tidje connected with i. Then by carefidly opening the cocks i and n alternately 
and manipulating the mercury jjump, the gas in the apparatus could be completely 
removed. When this was effected the mercury in the tubes h, g, and k stood at 
the same level. Before introducing the gas the bulb was heated in an air-bath for 
about an hoiir to 150° C. to move all traces of moisture, and was washed out several 
times with the diy gas. 
The operation of tilling the apparatus was the reverse of that of emptying it. The 
gas was admitted slowly by opening the stop-cock /, and at the same time mercuiy 
was admitted through the stoj)-cock n. The quantity of gas required to fill the 
apparatus to the desired pressure was at first found by trial, but after the first 
experiment it was known that when the level of the mercui'y in the tube h lay just 
ijelow the tube leading to i, the mercury rose to a certain height in tlie manometer 
tube g. When sufficient gas had been admitted the stop-cock i was closed and 
more mercury was allowed to enter the apparatus through //. The gas in the 
})ortion of tlie side tube aliove i. was easily displaced by mercury. (See also Part III., 
p. 171, rf 6'cy.) 
13. 21 te Scale. 
The measurement of the pressure in the thermometer bidb could have been easily 
and efiectively made liy employing a well constructed cathetometer with a standard 
scale attached to it. As, however, we had no means of obtaining such an instrument, 
Ave Avere oliliged to emiiloy the folloAving method. 
On the surface of a piece of plate-glass short scales ruled in millimetres by Zeiss 
of Jena, AA'ere cemented liy means of Canada balsam. The scale so constructed Avas 
fixed in front of the mercury column and dead-space, so that Avhen the loAvest scale, 
AA'hich AAvis 50 millims. in length, aauis in front of the dead-space, the four remaining 
scales AA^ere 100 millims. in length. With this arrangement it \A’as possible to fill the 
thermometer at three initial pressures, auz., at 350, at 500, and at 700 millims. of 
mercury, and to determine the pressure coefficient of the gas at each pressure. The 
method of observation Avill be described later. 
The distances betAveen the points on the scale at AAdiich obsei'Amtions Avei'e made 
AAmre first of all determined by us by means of a micrometer apparatus, designed by 
Mr. Hilger for measuring spectro-photographs, Avith a screAv of 1-millimetre pitch 
and a drum divided into 100 parts. By means of this instrument it AAas possible to 
determine lengths of 150 millims. to 0-002 milhm. At the end of our research the 
