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Proceedings of the Royal Society 
it stands on precisely the same theoretical footing as an air 
thermometer, or a mercury-in-glass thermometer, or an alcohol 
thermometer, or a methyl-butyrate thermometer, in respect to the 
graduation of its scale according to absolute temperature. Any one 
intrinsic thermoscope may be so graduated ideally by thermo- 
dynamic experiments on the substance itself without the aid of any 
other thermometer or any other thermometric substance ; but the 
steam-pressure thermometer has the great practical advantage over 
all others, except the air thermometer, that these experiments are 
easily realisable with great accuracy instead of being, though ideally 
possible, hardly to be considered possible as a practical means of 
attaining to thermodynamic thermometry. In fact, for water-steam 
it is only the most easily obtained of experimental data, the 
measurement of the density of the steam at different pressures, that 
has not already been actually obtained by direct experiment. 
Whether or not when this lacuna has been filled up by direct 
experiments, the data from water-steam alone may yield more 
accurate thermodynamic thermometry than we have at present from 
the hydrogen or nitrogen gas thermometer, — to be described in a 
subsequent communication to the Royal Society (Proceedings, 
April 19, 1880) — we are unable at present to judge. But when 
once we have the means, directly from itself, or indirectly from 
comparison with hydrogen or nitrogen or air thermometers, of 
graduating once for all a sulphurous acid steam thermometer, water- 
steam thermometer, or mercury-steam thermometer, that is to say, 
when once we have a table of the absolute thermodynamic tempera- 
tures corresponding to the different steam -pressures of the substances 
sulphurous acid, water, and mercury, we have a much more 
accurate and more easily reproducible standard than either the air 
or gas thermometer of any form, or the mercury thermometer, or 
any liquid thermometer can give. In fact, the series of steam 
thermometers for the whole range from the lowest temperatures can 
be reproduced with the greatest ease in any part of the world by a 
person commencing with no other material than a piece of sulphur 
and air to burn it in, 1 some pure water, some pure mercury, and 
1 Practically, the best ordinary chemical means of preparing sulphurous 
acid, as from sulphuric acid, by heating with copper, might be adopted in 
preference to burning sulphur. 
