RICHARDS AND MARK. — EXPANSION OP GASES. 419 



determinations of the expansion of a gas under constant pressure, and 

 Regnault's observations furnish still the only considerable data concern- 

 ing this condition. 



The Apparatus. 



The apparatus about to be described has several advantages over 

 others. In the first place all the gas is within the constant bath. Again, 

 a smaller range of temperature is used, thus involving less uncertainty on 

 account of the changing of coethcient of expansion with the temperature. 

 The two fixed points used were the melting point of ice, 0°, and the 

 transition temperature of sodic sulphate, 32.383°.* Lastly, the pressure 

 exerted by the gas can be certainly read to within the hundredth of 

 a millimeter of mercury and always under the same conditions, which 

 are entirely independent of atmospheric pressure. 



The last object is attained by comparing the pressure in a special reser- 

 voir (which is not open to the atmosphere and which is protected fiom 

 changes in temperature) with the pressure of the gas under investigation, 

 and then reading the pressure within this reservoir with the required 

 degree of accuracy by means of the admirable barometer devised by 

 Lord Rayleigh.t In this way the pressure can always be read under 

 the same conditions and entirely independently of the accidental state 

 of the atmosphere. It must, of course, be proved that the means of 

 comparing the pressure of the investigated gas and the pressure within 

 the reservoir introduces no new source of error. 



The arrangement which has been devised is as follows : The gas to be 

 experimented upon is contained in the bulb A (Figure 1), which is firmly 

 held in place in the bath B. In adjusting the apparatus at the lower 

 of the two temperatures (0°) the mercury is raised by means of the 

 levelling bulb V, wliich is carried by a finely threaded screw, until elec- 

 trical contact with tlie platinum point a is established. The establish- 

 ment of this contact between the platinum point and the mercury is 

 shown by the movement of a galvanometer needle. Preliminary experi- 

 ments proved that by this method a smaller change in the level of the 

 mercury could be detected than could be noted by observation through 

 the telescope ; that is, a change less than the hnndredth of a millimeter. 



* Am. J Sci., 6, 201 ; Zeit. pliys. Clicm.. 26, 000 (1808). A more accurate study 

 of tliis point iias been made recently, and very trustworthy data, giving its true 

 value in terms of the international standard, have been obtained. This investiga- 

 tion, by T. \V. Richards ami U. C. Wells, will be published in this Volume. 



t Lord Kayleigh, I'loc. Koy. Soc, 53, 103-138 (1893). 



