68 Mr Tregaskis on the Expansion of Vapour. 



AiiT. X. — On the Expansion of Vapour, By Richard 

 Tregaskis, Esq. of Perran, near Truro. Communicated 

 by the Author. 



XliXPERiMENTs on the elastic force of vapour in contact with 

 water at high temperatures are attended with difficulty, con- 

 siderable expence, and some danger. Hence few experiments 

 have been made on steam beyond the temperature of 343° of 

 Fahrenheit under a column of mercury. Even at this tem- 

 perature steam supports a column of mercury 20 feet in 

 height. 



The great difficulty attending experiments above this height 

 (which is equal to eight atmospheres) renders it particularly 

 desirable that some correct method be given for the calcula- 

 tion of force by temperature, founded on accurate experiments 

 made below it. 



Temperature and force increase, it is believed, in some 

 geometrical progression, but their ratios respectively have not 

 been published ; — perhaps they are not known. If they in- 

 creased in the same ratio, — if double the sensible heat would 

 generate exactly double the force, — there would be no difficulty 

 in calculation. But as the increase of force and temperature 

 are very different, a different ratio is required for each factor, 

 and the corresponding terms in each series should point out 

 the relative temperature and force. 



In order to this, some known fixed point is necessary for the 

 commencement of the scale. But zero of vapour, like that of 

 temperature, has not been fixed. The freezing point of water, 

 or rather the melting point of ice, naturally presents itself as 

 the zero of vapour ; but it has been placed lower on high 

 authority. Yet if the vapour of water has no existence till 

 fluidity is produced, it follows that the commencement of 

 fluidity is low enough ; for ice must be liquefied before it can 

 be vaporized. 



With this in view, I have examined various experimental 

 results on the elasticity of vapour, and compared the column of 

 mercury supported with the temperature required to maintain 

 vapour of sufficient tension to support the column. The result 



