RICHARDS AND MARK. — EXPANSION OF GASES. 423 



points a and a' was then observed by means of the micrometer-telescope 

 of an excellent Geneva cathetometer. The divisions of the micrometer 

 had previously been standardized by reference to various parts of an 

 accurate Geneva brass meter-stick placed at the same distance from the 

 telescope as was the bulb. F'or the sake of greater simplicity in cal- 

 culations, this distance (79 cm.) between the scale and the object lens of 

 the telescope was always adopted whenever the telescope was used. 

 Five determinations on different parts of the scale gave an average value 

 of one division of the micrometer screw dial as equal to 0.01248 mm. 

 In order to verify the measurement of the platinum points, the upper 

 pair were afterwards levelled, and the heights of the lower pair were 

 observed through the telescope. The following nine determinations 

 showed the distance between the points in the side tube to be greater 

 than that between those in the bulb by 0.90 ram., 0.89 mm., 0.86 mm., 

 0.88 mm., 0.89 mm., 0.88mm., 0.87 mm., 0.86mm., 0.87 mm.; average 

 0.88 mm. 



The bulb was then placed into its position in the bath, joined at d 

 by a rubber connector to the rest of the apparatus, and firmly clamped to 

 a massive horizontal iron rod. This part of the apparatus is not shown 

 in the diagram. It is absolutely essential that this clamping be as rigid 

 as possible, since a change in the position of the bulb between one set of 

 readings and another would be fatal to the accuracy of the experiment. 

 The ends of the horizontal rod were clamped to two firm iron supports 

 a meter apart. In order to cause a change in the observed relative 

 heights of the platinum points great enough to affect the result of the 

 experiment, one end of the rod would have to be raised five tenths of a 

 millimeter above the other end. In our apparatus, the unequal coeffi- 

 cients of expansion of the supports caused as a matter of fact a slight 

 displacement of this kind, which will be obviated in further work. An 

 empirical evaluation of this effect showed it to amount to 0.025 mm. (at 

 the points) per degree ; but this correction was often eliminated, since the 

 observations were made as much as possible at constant temperature, 

 when the bulb was unalterable in position. 



The bath consisted of a large inverted glass bottle, from which the 

 bottom had been cut off. It was surrounded by cotton, then by a coil 

 of lead pipe through which warm water was passed when the bath was 

 filled with sodic sulphate, and theu again by cotton. The whole was 

 enclosed in a wooden box, which served as a support and which was 

 securely screwed to the immovable desk upon which all the apparatus 

 stood. 



