42 HEAT. 



But we know that Boyle's Law is not, in fact, quite true, so that it 

 is necessary to investigate the two cases separately. 



The Volume Expansion at Constant Pressure Gay-Lussac's 

 Method. The earliest investigations having any approach to accuracy 

 were made by Dalton and by Gay-Lussac. One arrangement which Gay- 

 Lussac adopted for the purpose is shown in Fig. 33. 



Dry air or other gas was admitted to a bulb A through a drying- 

 tube, and a small mercury index i was introduced in the tube to indicate 

 the volume, and to cut off communication with the external air. The 

 bulb was then placed, with the stem horizontal, in a vessel of water, as 

 in the figure, the temperature being roughly indicated by the ther- 

 mometer t', and, more accurately, by the thermometer t, which could be 

 drawn out till the column of mercury was just visible. The air tube 

 could also be drawn out till the index i was visible. The vessel of water 

 was heated from below. The position of the index was observed at various 

 temperatures, and the bulb and tube having been previously gauged, the 

 air expansion could be found. Gay-Lussac arrived at a result which may 



Drying 



Tube 



FIG. 33. Gay-Lussac's Apparatus for Determination of Expansion of Gases 

 at Constant Pressure. 



be stated thus : whatever the gas, 267 volumes at increase by 1 

 volume for each rise of 1 temperature, becoming 367 volumes at 100. 



Dalton had, a short time previously, found that different gases 

 expand equally ; he gave the expansion as 1 in 483 volumes at 32 F. 

 for each rise of 1 F. This corresponds to 1 in 269 volumes on the 

 Centigrade scale. 



Later investigations have shown that the general conclusions of Dal- 

 ton and Gay-Lussac are nearly, but only nearly, true, their numerical 

 results giving too large an expansion, while the expansion is also found 

 to vary slightly for different gases. According to Regnault, to whom we 

 owe a very extensive series of researches, 273'1 volumes of hydrogen 

 at expand 100 volumes for a rise of 100, while the same expansion 

 is given by 272-4 volumes of air and 269 '6 volumes of carbon dioxide. 

 Other experimenters have found nearly the same values. It will be 

 sufficient if we here describe 



Regnaulfs Experiments. Some of Regnault's researches were carried 

 out by a method first used by Rudberg. The two parts of the 

 apparatus are represented in Figs. 34 and 35. The bulb A was 

 placed in a boiler, where it was surrounded with steam and then alter- 

 nately exhausted by the pump, and filled with dried air many times, till 

 all traces of moisture were removed. The taps tt were then turned, so 



