174 
MR. H. L. CALLENDAR ON THE PRACTICAL 
Comparison of the Coefficients of Expansion of Gases. 
The sulphuric acid gauge is specially adapted for effecting this comparison by a 
differential method. 
Similar thermometers, filled with different gases at approximately the same pressure, 
are exposed to the same source of heat and connected to the same mercury manometer, 
so that the external circumstances are precisely the same for both, and the difference 
of their pressures at any time is indicated by the sulphuric acid in the gauge. 
In a comparison of air and CCh between 0° and 100° the difference amounts to 
about 40 mm. of acid, a very measurable quantity. 
The Mercury Manometer. 
In the first experiments the manometer belonging to a “Jolly” air thermometer 
was used. It had a glass mirror scale of millimetres divided with considerable 
accuracy, but it was found that the tubes were of too small diameter, and the mercury 
was apt to stick appreciably. 
Another manometer was therefore constructed, with wider tubes, about 1'7 cm. 
internal diameter, which was found to be exceedingly satisfactory ; the same mirror 
scale was used, and the same adjusting apparatus. 
The scale is only divided to millimetres, but it is possible, with practice, to estimate 
with considerable accuracy to the tenth of a millimetre. It appears from the many 
observations made with the apparatus that the probable error of a single observation 
is of about this magnitude. It would have been theoretically preferable to take the 
readings with a kathetometer, but this would have involved great loss of time, and 
there was not one to spare. 
To avoid errors of parallax, Ac., readings were taken as follows:—The mirror scale 
was fixed vertically in a vertical plane, with the aid of a plumb-line. A telescope 
capable of sliding up and down, and being clamped at any height on a vertical axis, 
was levelled by focussing it on its own image in the mirror at the distance of about a 
metre ; this gave a magnifying power of about three diameters as compared with the 
naked eye at a distance of 10 inches. 
To eliminate personal errors in estimating the tenths of a millimetre, a series of 
independent observations at different points of the scale were taken at each constant 
temperature. The close agreement of these shows that the apparatus performed 
satisfactorily. 
The mirror scale was tested for scale errors by comparing it with the standard 
metre, with the aid of reading microscopes; the errors were found in all cases to be less 
than •01 cm., and are therefore neglected. 
In taking observations, the reading of the top of the meniscus in each limb is 
recorded and the difference set down in an adjacent column ; this is then corrected for 
temperature and reduced to zero, allowing for the linear expansion of the glass scale. 
