for Equal Volumes under Constant Pressure. 217 



defect, as compared with the actual value, must be so much the 

 greater the further the gas is from the perfectly gaseous state. 

 Since, now, both Clausius and Buff calculated the theoretical 

 specific heat for a molecule of n atoms too high by a quantity 



which may be expressed generally by the formula (n — 2) 



in the case of bodies which, like water, hydrosulphuric acid, 

 ammonia, (marsh-gas, ethylene*,) chloride of ethyle, alcohol, 

 acetone, ether, and sulphide of ethyle, are not far enough re- 

 moved from the state of perfect gases to counterbalance the errors 

 of the mode of calculation, the surprising result comes out that 

 the calculated specific heat is considerably higher than the ob- 

 served specific heat. In contrast with this, the specific heats cal- 

 culated according to my method for the perfectly gaseous condition 

 are never greater, to an extent worthy of consideration, than the 

 observed specific heats, as the above Table shows, — a circumstance 

 which speaks in favour of the views here developed, and which, 

 in view of many of the values calculated by Buff and Clausius, 

 I think ought to be specially pointed out. In accordance with 

 this, the specific heat of carbonic acid, which Regnaultf found 

 to increase with the temperature (a peculiarity which he consi- 

 ders it probable is shared by all the gases which are far from 

 the condition of perfect gases), is greater than that calculated by 

 my formula. Thus the lowest value observed was 0*2801, while 

 the calculated value is 0-272. 



§ 5. Minimum limit of specific heat. 



The applicability of the equation 7'= (ra + 5) . 0*034 to the 

 calculation of the specific heat of bodies with monatomic mole- 

 cules (of which mercury and cadmium are admitted to be ex- 

 amples) may be considered doubtful. It depends on the answer 

 that is given to two questions : — Do the molecules hitherto called 

 monatomic really consist of a single atom, homogeneous through- 

 out its entire mass ? and, Can we, in the case of such a molecule, 

 talk of an atomic as distinguishable from the molecular motion ? 

 The former question, for the present at least, does not admit of 

 an answer. But if it be answered in the affirmative, and the se- 

 cond in the negative, we get the expression 7 = 5 . , 034 = , 17, 

 which must be the value of the lowest conceivable specific heat 

 for the gaseous state. In any case, if it were experimentally 

 ascertained that mercury or cadmium possessed this specific 



* Neither of these gases could be obtained quite pure ; but still the im- 

 purities could not, according to Regnault's statements (up. cit. pp. 139 et 

 seq.), be sufficient to explain the great differences between the values cal- 

 culated by Buff and Clausius and those observed by Regnault. 



t Op. cit. p. 128, also p. 298. 



Phil. Mag. S. 4. Vol. 34 No. 229. Sept. 1867. Q 



