SECT. II.] 



PROPERTIES OF STEAM. 



459 + t' 



v = the volume at the temperature /'. 



RULE. As the volume the vapour occupies is to the volume it would become 

 by expansion, so is the elastic force at the lower temperature to that at the 

 higher one. 



Or, 



459 +S 459 



Taking as an example M. Cagniard de la Tour's experiments on ether, it is 

 stated that it was completely in a state of vapour at a lower degree ; but the 

 differences do not indicate this to have taken place till it was 447, and its force 

 was 68 - 8 atmospheres; required its force at 617. In this case 



459 + 617 



~\- 44 / 



ca , _ 

 x 68-8 = 81-7 atmospheres. 



In the experiment he states it as 94 atmospheres, and undoubtedly in conse- 

 quence of the vapour of mercury forming in the apparatus, (art. 107.) ; and a like 

 remark applies to all his experiments ; for our rule for the expansion rather 

 exceeds the truth than otherwise. 



120. By reversing the process, we may find the volume steam will occupy 

 under any compressive force not exceeding 60 atmospheres, when its volume is 

 known for a given temperature and pressure. For example, at 60 its force being 

 30 inches of mercury, its volume is 1324 times its volume in water. 1 Now by 

 increasing its temperature to the degree t' its volume would be, 



1 The volume of any vapour or gas at 60 and 30 in. is easily found from chemical tables con- 

 taining their specific gravity, compared with air at that temperature and pressure ; for air is 828 

 times the volume of an equal weight of water ; consequently, the number 828 being multiplied 

 by the specific gravity of the liquid, and divided by the specific gravity of the vapour in question, 

 gives its proportion of volume to an unit of volume of the liquid. 

 Thus steam is of the specific gravity '625 ; and 



828 



625 



= 1324. 



The following table may be found useful in similar calculations with various liquids. 



From this table it appears that one volume of water produces more vapour than an equal volume 

 of any other substance in the list. 



