322 Mr. Ivory's Answer to an Article by Mr. Henry Meikle 



I proceed next to Mr. Meikle's strictures on the explana- 

 tion, I have given, of the variation of temperature when air 

 changes its bulk. I have not seen any of the articles he cites in 

 the Edin. Phil. Journ. What I have written on this subject is 

 no more than an easy deduction from the usual theory of the 

 thermometer ; and the best way to enable the reader to judge 

 of the justness of Mr. Meikle's animadversions, is briefly to 

 explain the principles I proceed upon, disengaging them, as 

 far as perspicuity will permit, from all purely mathematical 

 calculations. 



In an air- thermometer, or, which is the same thing, in a 

 mass of air which changes its bulk by the application of heat, 

 the pressure being constant, I take it for granted that the ca- 

 pacity of the air for heat, within certain limits, remains inva- 

 riable; that is, I suppose that equal quantities of absolute 

 heat, produce equal variations of volume and temperature. It 

 will be admitted that this postulate is true, so long as two 

 thermometers, one of air and the other of mercury, both ex- 

 posed to the same flow of heat, continue to indicate the same 

 temperatures. To speak more particularly, but without aim- 

 ing at great precision, the capacity of air for heat may be sup- 

 posed constant between —40° and +300° on the centigrade 

 scale, comprising a range of temperature between 300° and 

 400°. The reasoning which follows is entirely founded on 

 this experimental fact, and it must be considered as applying 

 only within the limits mentioned. 



Suppose that the given mass of air has undergone a certain 

 variation of bulk ; and let us put h for the whole quantity of 

 heat which has produced this effect ; and t, for the change of 

 temperature in passing from the one volume to the other ; then 



we shall have — = k 9 k standing for some number not yet 



known, which however is invariably the same so long as the 

 temperature t is contained within the assigned limits. All 

 this is manifest from what is said above; for the equation 

 merely expresses that the absolute heat which causes any 

 change of volume is proportional to the variation of tempera- 

 ture, according to the postulate laid down, and as must be the 

 case if the thermometer be an exact measurer of heat. Now 

 h, whether it is heat added to, or abstracted from, the mass 

 of air, is always greater than t, the rise or depression of tem- 

 perature. For when heat is added, the volume of the air is 

 enlarged, and some heat is absorbed and disappears without 

 acting on the thermometer ; on the other hand when heat is abs- 

 tracted, the volume is diminished, and some heat is evolved 



which 



