530 ME J. Y. BUCHANAN ON 



steam so proceeding had the same temperature as the boiling solution, because when 

 steam is blown into such a solution it is condensed until the solution has been heated to 

 its boiling point. If the solution can condense outside steam, it necessarily must be able 

 to condense its own steam, for the two substances are identical. If the bulb of a ther- 

 mometer is immersed in brine boiling over a flame, it is irregular in its readings. The 

 irregularities are due to superheating of the liquid consequent on the adhesion of the 

 solution to the glass or metal. The ebullition is never quite regular, and the reading of 

 the thermometer varies, showing a fall of temperature when steam is coming off freely, 

 and a rise when there is intermission. It is very difficult in this way to obtain 

 exact observations of the boiling temperature of a solution, or indeed of pure water. 



On the other hand, it is very easy to get results of any desired degree of exactitude 

 by using an abundant current of saturated steam to boil the water or brine. Over- 

 heating is impossible, and bumping equally so. A thermometer of any degree of deli- 

 cacy may be used ; there is never any room for doubt that its temperature is, to tiie 

 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 tempera- 

 ture falls at ouce 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 impossible for the steam, after it has passed through the mixture of salt and 

 brine, to have a pressure above that of the atmosphere 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 ther 

 mometer keeps the temperature with absolute steadiness until so much steam has con- 

 densed and so little solid salt remains that it cannot be assumed that every particle ot 

 steam condensed can immediately find a particle of salt to dissolve. Then the tempera- 

 ture begins to fall in the same way as that of a freezing mixture begins to rise. Operat- 

 ing in this way it is possible to obtain definite and perfectly constant temperatures with 



