4 Mr. Ivory on the Lcncs of the Condensation and Dilatation 



perties of air. An investigation instituted on the principles 

 I have endeavoured briefly to explain, promises at least some 

 degree of success. 



But a preliminary investigation seems necessaz-y. In what 

 manner do the elasticity and density of a mass of air depend 

 upon the heat combined with it in a latent form, and the heat 

 of temperature sensible to the thermometer? It is evident 

 that the latter quantities completely ascertain the condition of 

 a given body of air. If both of them remain constant, no 

 change can take place in the air's elasticity or density. If 

 both, or either of them vary, an alteration of the elasticity or 

 density, or of both together, will necessarily ensue. In the 

 present communication I confine myself to this preliminary 

 inquiry. 



We have no knowledge, in any instance, of the absolute 

 quantities of the heat of combination, or of the heat of tem- 

 perature ; we can reason only about the differences that occur 

 in the same body placed in different circumstances. For this 

 reason it is always necessary to refer the air, or gas, under 

 consideration, to some original or standard condition. I shall 

 suppose that the elasticity and density are each originally equal 

 to unit ; and I shall put p for the relative elasticity, or pres- 

 sure, and g = 1 + 0) for the relative density, in any other state. 

 Thus if h be the barometric pressure of the original body of 

 air, and h' the like pressure when the air has varied in any 



h' • ... 



manner, then — = p ; and if D and D' be the densities m 



h ^ 



the same two states, then ^j- = ^ = 1 + w. The original 



temperature being t, and the difference of temperature fl, the 

 actual temperature will be t + 9. If « denote the dilatation 

 for one degree of the thermometer, we have this fundamental 

 equation between the quantities defined, viz. 



and I shall call the expression by which j is multiplied the 

 factor, or the fimction, for temperature. Lastly, I shall de- 

 note by + i the degrees of the thermometer evolved or ab- 

 sorbed, when air or a gas passes from the original to any 

 other state ; reckoning positive the heat which becomes sen- 

 sible to the thermometer when air is condensed, and negative 

 the heat which disa}ipears when air is dilated. 



1. Taking the air at the surface of the earth as the stand- 

 ard for comparison, let a given mass of it be contained in a 

 close vessel; then let the dimensions of the vessel be suddenly 

 enlarged, allowing the air within to dilate till ihe density is 



diminished 



