72 
steam rapidly expands in volume for any increase of tem- 
perature beyond the temperature of saturation, it would, in 
any case, be impossible to decide by the unaided eye the exact 
temperature at which all the water became vapourized. The 
slightest error in deciding the temperature of saturation w^ould 
vitiate the experiments and render the results of no value. 
The difficulty thus resolves itself into finding some other 
test sufficiently delicate to determine the point of saturation. 
This has been overcome by what may be termed the saturation 
gauge, and it is in this that the novelty of the present experi- 
ments consists. 
The simplest form of the apparatus might be described as 
composed of two glass globes connected by a bent tube con- 
containing mercury, as in the differential air thermometer. 
Supposing a weighed portion of water introduced into one of 
the globes, and any larger portion into the other, and the 
apparatus placed in a bath and slowly heated, the pressure in 
each globe would increase equally up to the point at which 
all the water in one globe became vapourized. As soon as 
the satui-ation point in this globe was reached, the mercury 
column, up to this time level, would rise in the stem of that 
globe in which the pressure had almost ceased to increase. 
The instantaneous change of the position of the mercury is 
the indication of the temperature of the steam when saturated. 
Thus, at 290° Fahrenheit, the mercury column would rise 
nearly two inches for every degree above the saturation point, 
as the increase of pressure arising from vapourization is twelve 
times that arising from superheating or expansion, and a 
similar difference exists at other temperatures. 
The Author described one of the forms of apparatus by 
means of a diagram. It consists of a glass globe with a long 
stem placed in a copper boiler, fitted with blow-off cock, 
steam gauge, thermometer, and gas jets for heating. The 
stem of the globe passes dowm a glass tube depending from 
the boiler, and forms the saturation gauge. 
