CHEMICAL AND PHYSICAL NOTES. lol 
the vapour of water is reduced, not only by lowering its temperature, 
but also, while the temperature is kept constant, by dissolving any 
salt in it. The tension of the vapour of pure water at 100° C. is 
760 mm. If a small quantity of common salt or chloride of sodium 
be dissolved in it, the tension of its vapour is no longer 760 mm. 
at 100° C.; it is necessary to raise it to a higher temperature in 
order that its vapour may attain this tension. In proportion as more 
salt is added to the water, the higher is it necessary to raise the 
temperature of the water, or rather the resulting saline solution, in 
order to attain a tension of 760 mm. But there is a limit to the 
amount of salt which water can dissolve when boiling under a given 
pressure. When this amount has been added and dissolved, the 
solution is saturated and, so long as the atmospheric pressure remains 
the same, it is impossible to raise its boiling-point any higher. If 
heat is supplied, so as to keep the solution boiling, steam escapes 
from its surface, and crystals of salt separate out in the solution. 
The condition of the boiling solution is now precisely analogous with 
that of the freezing solution when, in the process of cooling, the 
cryohydric temperature and concentration have been reached. The 
temperature of the boiling solution remains constant, and steam and 
salt quit it part passu. In the freezing solution the salt and ice are 
both solid, and remain associated in the cryohydrate. In the boiling 
solution the salt is solid and the steam is gaseous, and they part 
company of themselves. 
The fact that steam produced by water boiling at 100° C., which 
under ordinary circumstances can produce by its condensation no 
higher temperature than that at which it was generated, should be 
able to raise the temperature of saline solutions many degrees above 
this temperature, appears at first sight paradoxical, and it was, in 
fact, largely disbelieved, in spite of the simplicity of the experiment 
to demonstrate it. 
In a paper already referred to, the use of ice melting in saline 
solutions of definite nature and strength was strongly recommended 
as affording an absolute thermometric scale for such temperatures. 
Extending these researches to steam and brines, the writer found 
that the condensation of steam in saline solutions could be used for 
fixing, or verifying, temperatures above the boiling-point of water. 
Also steam water, and salt can be used to form boiling mixtures, 
exactly as ice, water, and salt are used to form freezing mixtures ; 
and an independent and absolute thermometric scale is produced 
for high temperatures, just as with ice we have one for low 
temperatures. 
