46 VAPORIZATION. 



the water, since it is neither indicated in the steam nor in the 

 water ? It enters into the water, and converts it into steam, 

 without raising its temperature. As much heat disappears as is 

 capable of raising the temperature of the portion of water con- 

 verted into steam 1000 degrees, or what is the same thing, as 

 would raise the temperature of one thousand times as much 

 water by one degree. This is now generally assumed to be the 

 amount of the latent heat of steam. Dr. Black found it to be 

 about 960 degrees, Mr. Watt 940 degrees, and Lavoisier rather 

 more than 1000 degrees. 



Several circumstances may be remarked during the occur- 

 rence of this change in water. On heating water gradually in a 

 vessel we first observe minute bubbles to form in the liquid 

 and rise through it, which consist of air. As the temperature 

 increases, larger bubbles are formed at the bottom of the vessel, 

 which rise a little way in the liquid, arid then contract and 

 disappear, producing a hissing or simmering sound. But, as the 

 heating goes on, these bubbles, which are steam, rise higher and 

 higher in the liquid, till at last they reach its surface and escape, 

 producing a bubbling agitation, or the phenomenon of ebullition. 

 The whole process of boiling is beautifully seen in a glass vessel. 

 It will be remarked that steam itself is invisible ; it only appears 

 when condensed again into minute drops of water by mixing 

 with the cold air. 



It was first observed by Guy-Lussac, that liquids are con- 

 verted more easily into vapour when in contact with angular 

 and uneven surfaces, than when the surfaces which they touch 

 are smooth and polished. He also remarked that water boils 

 at a temperature two degrees higher in glass than in metal ; 

 so that if into water, in a glass flask, which has ceased to boil, 

 we drop a twisted piece of cold iron wire, the boiling is re- 

 sumed : it is only in vessels of metal that the boiling point is 

 regular, and should be taken in graduating thermometers. It 

 has lately been remarked by Mr. Scrymgeour, of Glasgow, that 

 if oil be present with water, the boiling point of the water is 

 raised a few degrees, in any kind of vessel. The reason why 

 water, in these circumstances, does not pass into vapour at its 

 usual boiling point, is not distinctly understood. The water 

 appears to be in a precarious state of equilibrium, as in the 

 other analogous case, when cooled with caution in a smooth 

 glass vessel considerably under its usual freezing point. The 



