172 
MR. H. L. CALLENDAR ON THE PRACTICAL 
observation is perfectly confined, and the same mass of air may be kept under obser¬ 
vation for a lengthened period, and is maintained perfectly dry. 
Since the bore of the H 3 S0 4 gauge tube is small compared with that of the mercury 
manometer, a change of pressure of air in the bulb first takes effect in the small-bore 
U gauge, and only alters the reading of the wide tube mercury manometer very 
slightly : this enables the instrument to be used when the temperature is not abso¬ 
lutely constant, as the reading of the U gauge can be instantly recorded, and the 
mercury manometer read at leisure. It might be thought that errors would be intro¬ 
duced by capillarity and viscosity of the acid in so small a tube; but it can be easily 
shown by direct experiment that this is not the case to an appreciable extent. If an 
open gauge be connected to a mercury manometer, the acid exactly and perfectly 
follows every readjustment of the manometer, and its capillarity and density may 
thus be easily determined in terms of mercury. 
It has been suggested that the acid might have some action on the confined mass 
of air, either absorbing it or giving off S0 3 : it would be difficult to prove that this is 
not the case. On the other hand there does not seem much reason to believe that 
it is.* 
One great advantage of the H 3 S0 4 gauge is that, when the instrument is at constant 
temperature, such as that of melting ice, by altering the adjustment of the mercury 
manometer, the reading of the IhS0 4 gauge is altered consistently, and a great 
number of quite independent observations may be taken, which ought, of course, on 
induction, to lead to almost identical results, the mean of which will be to a great 
extent freed from scale and personal errors. 
The H 2 S0 4 gauge is provided with a millimetre scale, and is carefully calibrated; it 
is difficult to make an error of '02 centim. in reading the scale, and all errors are 
divided by about 7 in the reduction to mercury, so that they may practically be 
neglected as far as the pressure is concerned; they become more important in 
applying the volume correction, especially if the volume of the bulb be small. 
The extremely simple sort of gauge shown in the diagram may be used with great 
advantage, but it is liable to some slight inconveniences. 
Supposing the air in the bulb to be at atmospheric pressure and temperature, a 
variation of 1° C. in its temperature alters the reading of the H 3 S0 4 gauge by nearly 
2 centims. If the limb of the gauge be only 10 centims. long, it has thus a range of 
only about 5° without readjustment of the mercury manometer, so that, when the 
temperature is changing, the manometer requires to be continually readjusted. 
More elaborate forms were, therefore, actually used in most of the experiments, 
one of which is shown in fig. 2, Plate 11. 
The gauge tube carries a bulb B which is calibrated, and whose volume is about 
* A very complete answer to this and other objections to the use of sulphuric acid will be fouud in 
§ 18 of Sir William Thomson's article on “Heat,” in the ninth edition, 1880, of the ‘ Encyclopedia 
Britaunica,’ vol. 11, p. 561. 
