235 
ARTICLE IX. 
On the Force requisite for the production of Vapours. 
By Prof. Gustav Maanus*. 
[From Poggendorff’s Annalen, 1844, No. 2.] 
IT HAVE previously mentioned at page 226 that the water in 
the arm of the U-shaped tube a 6 d which had been well-boiled 
did not generally pass into the form of steam until it was under 
a pressure by several inches mercury lower than that which cor- 
responded to the expansive force of the vapours at the tempera- 
ture existing, but that then the formation of steam took place 
suddenly and with great violence. This phenomenon had, as I 
subsequently found, been already observed by Wattt, and after 
him by Southern, in precisely the same way in the barometer 
tubes which they employed in their experiments. But neither 
have they, nor has any one since, drawn any conclusion from 
this phenomenon. 
Evidently the force necessary for the disengagement of the 
steam was greater than the expansive force of the steam at the 
existing temperature. If we were to admit that the formation 
of steam had been rendered difficult, owing to the attraction be- 
tween glass and water, it cannot be conceived why in such case 
the parts of the water did not separate from each other, the more 
so as it was always present in such quantity and constantly occu- 
pied so large a space, that the attraction of the glass could not 
possibly have extended to its inner parts. 
It therefore remains only to admit that the force which is re- 
quired for the production of steam is larger than its expansive 
force, or the force which it requires for its existence, because the 
cohesion of the fluid upon which its liquid condition rests must 
be overcome. 
It has already been asserted that the cohesion must be done 
away with in the formation of steam, but it has been regarded 
as very inconsiderable and has been neglected. On that account 
no one has deduced the conclusion from it, that the particle of 
liquid to be converted into vapour must constantly possess a 
* Translated by W. Francis, Ph.D., F.L.S. 
+ Robison’s System of Mechanical Philosophy, ii. pp. 31 and 170, 
