EXPEKIMENTAL KNOWLEDGE OF THE rEOPEETIES OF MATTER. 101 



results of Regnault's vpork have been confirmed by his successors within 

 the same range of pressure. 



In tome xsvi. of the Memoires, Regnanlt, in the third part of the first 

 memoire of the volume, determines the variations of vp for jjressures 

 from about one atmo up to not more than eight atmos for atmospheric 

 air and for carbon dioxide again ; and for oxygen, nitrous oxide, nitric 

 oxide, carbon monoxide, mai'sh-gas, cyanogen, ammonia, hydrochloi'ic 

 acid, hydrogen sulphide, sulphur dioxide. The variations of vp for 

 atmospheric air and for carbon dioxide agreed very well with the values 

 obtained up to 8 atmos in the former set of experiments ; and this agree- 

 ment is a guarantee of the general accuracy of the results for the other 

 gases, the gas in each case being tested and found pure and experimented 

 on at some fixed temperature lower than 10°. 



The deviations from Boyle's law were in all these cases found to be in 

 direction of greater compressibility. It should be mentioned that the 

 dates of publication of these two sets of memoires of Regnault were 1847 

 and 1862. 



The volumes, in each case, occupied by the gas at different pressures 

 were known by the weight of mercury corresponding to the part of the 

 manometer-tube occupied by the gas, and the pressure by the vertical 

 height of mercury supported by the gas, this height being corrected for 

 standard pressure and density of mercury : for Regnault found by special 

 experiments that mercury is compressible, and the density which at 0° 

 is 13'596 is at higher temperatures less, according to the rate of absolute 

 expansion of mercury. The results of these investigations are given in 

 the fifth and seventh memoires of tome xxi. ; at p. 328 there is a list of 

 absolute dilatations of mercury between 0° and T°, where T is 10°, 20°, 

 or any multiple of 10° up to 350° ; the total dilatation for 



0° to 10° being -001792 for 1 volume at 0° 

 0° „ 350° „ -065743 „ 



and the compressibility of mercury per atmosphere was investigated and 

 found (p. 462) to be -00000352, and for this corrections have to be applied 

 throughout the height of the column of mercury. It will be noticed how- 

 ever that for pressures of not more than 30 atmospheres the correction 

 due to this is inappreciable, and will in fact be drowned in inevitable or 

 unavoided sources of error. The constant temperature of the gas in the 

 manometer was always secured by surrounding it with a jacket through 

 which a current of water of constant temperature was passing constantly. 



Regnault, as Debray pointed out (see Ditto's ' Proprietes generales 

 des corps,' p. 17), omitted from his calculations the weight of the com- 

 pressed gas in the manometer, which being added increases the apparent 

 pressure of the gas, the increase in Regnault's tables being due to the 

 weight of a column of about 2 metres of the gas : a small correction is due 

 to Regnault's numbers on this account. 



Among the data required for the determination of the values of pu 

 mention must not be omitted of the atmospheric pressure which must 

 be reckoned and added to the pressure of the column of mercury. 



This outline must suffice to give some notion of the precautions taken 

 to avoid error, and the pains taken to secure an accurate determiaa- 

 tion of all the data required in these investigations, in which Regnault 

 obtained for many gases up to eight atmospheres, and for a few up to 

 thirty, the actual relations of pressure and volume, and the extent to 



