On Steam and Brines 153 



impossible, and bumping equally so. A thermometer of any 

 degree of delicacy may be used ; there is never any room for 

 doubt that its temperature is, to the minutest fraction of a 

 degree, the same as that of the boiling liquid, and the value 

 of the observation thus depends only on the trustworthiness 

 of the instrument. In the' paper on 'Ice and Brines' above 

 referred to, it was pointed out that the melting-point of ice 

 in brines of determined nature and strength could be used as 

 fixed points on the thermometric scale, in the same way as 

 the melting temperature of ice in pure water is habitually 

 used. Similarly the condensing temperature of steam in 

 saline solutions forms a ready means of fixing exactly certain 

 points above the ordinary boiling-point of water. Also the 

 minimum temperature, or cryohydric point, of freezing 

 mixtures is very useful, many of them being as well defined 

 as the melting-point of ice in water. 



Our boiling mixtures occupy a similar position with 

 respect to the boiling-point of water that the freezing 

 mixtures do to its freezing-point. In freezing mixtures the 

 dry salt is mixed with pounded ice or snow ; if the mixture 

 is properly made, the temperature falls at once to the true 

 minimum, and remains quite steady for a great length of 

 time. In boiling mixtures the dry salt is placed in a U-tube 

 of special dimensions, to be described presently, and steam 

 is passed into it by one leg, while the other leg carries the 

 thermometer, and the surplus steam escapes through a side 

 tube. The supply of steam must be abundant, while the exit 

 tube for the steam must be sufficiently wide to make it im- 

 possible for the steam, after it has passed through the mixture 

 of salt and brine, to have a pressure above that of the atmo- 

 sphere into which it exhausts. From the time when enough 

 steam has condensed to form a more or less liquid magma 

 through which the steam bubbles, the mass is kept thoroughly 

 well mixed, and the thermometer keeps the temperature with 

 absolute steadiness until so much steam has condensed and 

 so little solid salt remains that it cannot be assumed that 

 every particle of steam condensed can immediately find a 

 particle of salt to dissolve. Then the temperature begins 



