RICHARDS AND MARK. — EXPANSION OF GASES. 425 



the iron rod of the gauge was made with great care ; and finally the dis- 

 tance between the meniscus m and the point n was read through the 

 telescope. It was found possible to be certain of this very important 

 reading within 0.01 mm. when the illumination was properly arranged. 

 The pi'essure in the upper part H of the gauge was immediately taken 

 by means of the McLeod gauge G — the reading of which multiplies the 

 real pressure by fifty — and also the temperature of the Lord Rayleigh 

 barometer was observed by means of two thermometers placed one at its 

 top and one at its base. It is necessary that this temperature be known 

 to within the tenth of one degree, in order to correct for the unequal 

 expansion of the mercury column and the iron rod. To prevent sudden 

 changes of temperature the barometer was incased in a wooden air- 

 jacket, the lower iron point being observed through a glass window. 

 When the room containing the apparatus varied rapidly in temperature, 

 the jacket was not always adequate for its purpose, hence the observations 

 were usually made when the room was at constant temperature. After 

 the first setting, which involved the admission of the suitable amount of 

 hydrogen into the bulb, subsequent determinations consisted merely in 

 adjusting the pressure in the side tube so as to make simultaneous con- 

 tacts at the platinum points, and in making the readings on the Lord 

 Rayleigh barometer. This process took about five minutes and was 

 repeated at intervals during the day. When the temperature of the 

 room remained nearly constant, these readings were very uniform, show- 

 ing that the setting of the instrument could be accomplished with sufficient 

 accuracy. For example, three successive readings in a typical case were 

 L30 mm., 1.31 mm., and 1.30 mm. 



The readings at the lower temperature were made in every particular 

 in the same manner as those just described. The purity of the ice in the 

 bath was tested by the temperature of melting, recorded by the same 

 thermometers as were used in the sodic sulphate, and also by the anal- 

 ysis of the water coming from the melted ice. A complete observation, 

 then, consisted of one series of readings at high temperature, and a 

 similar one at low temperature. It will be noticed that the essential 

 part of the experiment consisted in the observation of slight changes in 

 pressure, measured in such a way that any error tends to eliminate itself. 

 These changes were so small that Boyle's law may safely be used in 

 referring them to changing volume. 



In calculating the coefficient of expansion, let a equal the expansion 

 per degree centigrade of unit volume at 0°C. Let K equal the volume 

 at 0°C ; V the increase in volume ; t the difference in temperature between 



