ON THE EXPANSION OF ELASTIC FLUIDS. 133 



this amount will take place in common air, when not freed 

 from aqueous vapour, which was the fituation of all my 

 factitious gafes, 



Upon the whole therefore I fee no fufficient reafon why we The law Is gc- 



may not conclude, that all eiaflic fluids under the fame preffure* 1 *™}.* **}* 



J . , , , , . r . elaftic fluids. 



expand equally by heat, — and that for any given. expanfion -of mer- 

 cury, the corresponding expanfion of air is proportionally fo?ne- 

 thing lefs, the higher the temperature. 



This remarkable fact that all elaftic fluids expand the fame Suppofed caufe 

 quantity in the fame circumftances, plainly thews that the ex- they have to 

 panfion depends Jblely upon heat : whereas the expanfion in a « r aftion. inS 

 (olid and liquid bodies feems to depend upon an adjuftment 

 of the two oppofite forces of heat and chemical affinity, the 

 one a conjlant force in the fame temperature, the other a va- 

 riable one, according to the nature of the body ; hence the 

 unequal expanfion of fuch bodies. It feems therefore that 

 general laws refpecling the abfolute quantity and the nature 

 of heat, are more likely to be derived from elaftic fluids than 

 from other fubftances. 



In order to explain the manner in which elaftic fluids ex- Hypothefis to 

 pand by heat, let us affume an hypothefis that the repulfive explain the law 

 force of each particle is exactly proportional to the whole ' 

 quantity of heat combined with it, or in other words to its 

 temperature reckoned from the point of total privation : then, 

 iince the diameter of each particle's fphere of influence is as 

 the cube root of the fpace occupied by the mafs, we (hall 



3 3 



Jiave/lOOO : / 132.5 (10 : 11, nearly) :: the abfolute 



quantity of heat in air of 55° : the abfolute quantity in air 



of 212°. This gives the point of total privation of heat, or Singular coinci- 



abfolute cold, at 1547 ° below the point at which water freezes. d * nce as to tb ? 



Dr. Crawford (On Animal Heat, &c. page 267) deduces the 



faid point by a method wholly different to be 1552°, — So near 



a coincidence is certainly more than fortuitous. 



The only objection I fee to this hypothefis is, that it necef- An objeclion if 



farily requires the augmentation of elaftic fluids for a given w e admit the ex- 



\.\ ri.,1 •,,., i panfion of mct> 



quantity of heat to be greater in the higher temperatures than cur y to be as the 



jn the lower, pecaufe the cubes of a feries of numbers in heat > but noc fo 



arithmetical progreffion differ more the larger the numbers or thTuw'bf ill' 



roots: but it has juft been (hewn that in fact an augmentation other -fluids. 



of a contrary kind is ojbfervrd. This refers us to the con- 



fider- 





