﻿164 Mr. C. Tomlinson's Historical Notes on some 



clearly made out during the fine experiments undertaken by Dal- 

 ton, Watt, Robison, Southern, and others for determining the 

 pressures of saturated steam at different temperatures above and 

 below the standard boiling-point. It was noticed that if a mi- 

 nute portion of soda, or of some salt soluble in water and not 

 capable of rising in vapour with it, be allowed to ascend to the 

 top of the mercury, the column rises, thereby indicating a dimi- 

 nished pressure of steam, although the soda has not touched it, 

 but remains covered by the layer of water on the top of the mer- 

 cury. This shows that the elasticity depends not merely on the 

 temperature and the nature of the vapour, which are both un- 

 changed, but on the nature of the liquid. The adhesion of the 

 soda to the water tends to restrain the water from evaporating, 

 and this tendency is a measurable force and here measured ; for it 

 partly balances the tension of the water, or its tendency to emit 

 steam, and thus makes the steam-emitting tension of a solution 

 of soda measurably less than that of pure water at the same tem- 

 perature. As the difference remains at all temperatures, the 

 solution must always be made hotter than pure water in order 

 to give steam of the same elasticity. 



7. The effect of air dissolved in the water on the boiling-point 

 was noticed in minute detail by De Luc* in 1803, not indeed 

 for the first time; for in his previous works, published in 1772 

 and 1786f, he had described the principal experiment on which 

 his remarkable theory was based. He says: — "Lephenomene 

 de Pebullition est produit par des bulles d'air que la chaleur 

 degage du liquide . . . . ; quand on a prealablement purge Peau 

 de tout Pair qiPelle contenoit, elle ne peut plus bouillir; et la 

 raison en est que les vapeurs ne peuvent se former qu'a des 

 surfaces libres. Les bulles d'air qui se rassemblent dans son 

 sein y produisent des solutions de continuite; c'est-a-dire, ces 

 surfaces libres necessaires ; mais quand Peau est purgee d'air les 

 vapeurs ne peuvent se former qiPa sa surface exterieure " {Intro- 

 duction, &c. vol. i. page 247). De Luc had already described, in 

 1772, with great minuteness of detail an experiment in which a 

 matrass containing water, and also a small thermometer, had the 

 upper part of its tube drawn out into a capillary bore, and, the 

 matrass being heated in hot water, the air as it accumulated in 

 the fine tube was got rid of by an ejection of steam, and the 

 tube was sealed. The tube was also subjected to percussion 

 during a long time, and the process of heating was continued 



* Introduction a la Physique terrestre par les Fluides expansibles. 

 Paris, 1803. 



t Recherches sur les Modifications deV Atmosphere. Geneva, 1772. See 

 chapter 10 of the Supplement to vol. ii. Recherches sur les Variations de la 

 Chaleur de VEau bouillante. Idees sur la Meteorologie. London, 1786. 



