on Evaporation 4 1 ? 
the air is at last almost completely exhausted, and the re- 
maining pressure acting on the manometer is that of steam. 
Mr. Nairne has shewn me an instance of that effect, in 
which the quantity of air indicated by the pear-gage, was 
not above r - b -‘ 0 - 0 ' part of the whole. That remarkable fact 
will prove, that in the following experiments, air itself was 
exhausted to the utmost degree, and thus reduced to nought 
in respect of any influence on the main phenomena. 
I must also premise, whence proceed some anomalies which 
will be observed in those experiments. It is in general from 
the nature of steam, from which, in our experiments, it 
is submitted to certain modifications, depending on its laws, 
but which cannot be always referred to them by immediate 
explanation, because of undetermined effects of the sides of 
the vessels, and of irregularities in the temperature. 
The law of steam, which is the most subject to anomalies 
by those causes, is that of two distinct maxima, one of pro- 
duction, the other of preservation. Those two maxima coin- 
cide, when the temperature is near the freezing point ; as in that 
case, the whole quantity of steam that can exist at one time 
in a space, is also produced when there is a sufficient quantity 
of water in any part of it ; whereby the hygrometer stands 
at 100, which is its point of extreme moisture. But in pro- 
portion as the whole apparatus grows warmer, the maximum 
of production falls short of that of preservation ; the quan- 
tity of steam does not increase, though it might be more dense 
without undergoing a decomposition ; and thereby the hygro- 
meter recedes from extreme moisture. 
Such is the general law of steam, as it results clearly from 
the whole of my experiments ; but in particular cases, it is 
