MEASUREMENT OF TEMPERATURE. 
171 
about twenty times as important in a mercury thermometer, where they produce 
notable changes of zero. One great advantage of the air thermometer is that they 
can usually be neglected. 
The bulb is connected with the U gauge by a tube of capillary bore, part of which 
is heated with the bulb, and part of which is at the known atmospheric temperature, 
a length of about 5 centims. being at intermediate and unknown temperatures. If 
the bore be between three and five-tenths of a millimetre, and the capacity of the 
bulb 50 cub. centims., the uncertain error will be much less than TooUm an d may 
be neglected. 
Some kinds of glass are liable to be acted on chemically by the gas at high 
temperatures. For instance, hydrogen or coal gas exerts a powerful reducing action 
on lead glass at a dull red heat, and the action proceeds more slowly at much lower 
temperatures. Perhaps nitrogen is, for this reason, the most suitable thennometric 
material. Water vapour and carbon dioxide also are well known to exert a dis¬ 
integrating action, and are for other reasons also especially to be avoided. 
The Sulphuric Acid Gauge . 
It has usually been the custom to confine the air to constant volume in the air 
thermometer by a column of mercury whose level is adjusted up to a fixed mark 
before taking an observation. The use of mercury necessitates a very wide tube, 
otherwise the effects of capillarity produce large errors: the volume of the air 
unheated must, in consequence, be large and also variable with the shape of the 
meniscus. Moreover, for accurate work, it is necessary not only to thoroughly dry 
and clean the bulb of the air thermometer, but also the tubes and mercury of the 
manometer which is in direct connexion with it. This it is practically impossible to 
do perfectly. Again, it is in most observations exceedingly inconvenient to have a 
mercury manometer rigidly connected with the bulb of the air thermometer, and in 
such close proximity with the source of heat that the mercury becomes unequally 
heated. The chief advantage of mercury is that its vapour tension is so small (about 
’001 centim. at ordinary temperatures) that it is negligible. Pure 1I.,S0 4 , however, 
possesses this same qualification, and has none of the above disadvantages. Since it 
wets the tube, the correction for capillarity is quite constant. If the bore of the tube 
be about 0'2 centim., the capillary elevation is about 0'5 centim. With a gauge 
tube of this bore the correction volume may be made quite small, and may be very 
accurately known, the shape of the meniscus being constant. This enables us to use 
bulbs of smaller capacity, which are, for many purposes, more convenient, without 
unduly increasing the magnitude of this correction. The use of sulphuric acid in the 
gauge tube is also attended with several other advantages. The air thermometer is 
thus rendered very sensitive, the slightest change of temperature is at once indicated, 
and it is very easy to see when the temperature is steady. The mass of air under 
z 2 
