402 
Mr. de Luc 
fluid, the object of hygrology and hygrometry, I shall give 
the name of steam, in every case. 
2. As long as steam exists, it has a power of pressure, as 
air itself ; but it does not belong to the class of permanent 
fluids, for it may be decomposed by a certain degree of pres- 
sure, or cooling, according to determined laws. 
3. The characteristic property of steam, from which pro- 
ceeds its decomposition by those causes, is, the having a fixed 
maximum of density by a given temperature, which increases 
with the temperature. Thus, when that fluid is arrived at 
the maximum correspondent to a certain temperature, it must 
undergo a decomposition, either by cooling, because its actual 
maximum is too great for the new temperature ; or by an 
increase of pressure (the temperature remaining the same), 
because its density becomes too great for that temperature. 
4. The degree of pressure exercised by steam, or which it 
can support without decomposition, being, cceteris paribus, 
proportional to its density, depends, in the same manner, on 
temperature. 
5. Steam is formed by every temperature, when a previ- 
ous space permits its expansion ; but, according to the above 
law, no steam can be formed when, for its formation, it ought 
to repel an obstacle superior, in the smallest degree, to the 
maximum of its power by the actual temperature ; and if it 
* 
has been formed under a pressure not exceeding its power, if 
that pressure increases, or the temperature lowers, ever so little, 
it is totally decomposed. 
6 . This is what determines both the degree of heat by 
which water can begin to boil, and the variations of that 
degree, by those of pressure. For ebullition is that state of 
