246 



Scientific Proceedings^ Royal Dublin Society. 



of a tliermostatieally controlled air, oil, or sand bath provided with a suitable 

 fan. With such an arrangement an evaporation which wOuld talve a day with 

 existing apparatus could be completed in an hour, so that the saving in time 

 would amply repaj^ any extra trouble in setting up the bath in the first instance. 



The main difficulty in evaporation, as will be seen from the foregoing, is to 

 supply the heat to the liciuid sufficiently rapidly to produce rapid evaporation 

 and yet prevent ebullition. The latter is to be avoided on two grounds : (1) its 

 very existence shows that the liquid is being superheated locally; and (2) it may 

 lead to loss of the liquid by spirting. 



Wlien liquid is heated in a vessel with only the convection currents to keep 

 it mixed, the bottom of the vessel may become heated much above tlie boiling 

 point of the liquid, and, since the convection currents are unable to mix the 

 liquid sufficiently rapidly, ebullition of .more or less violence occurs if the heat is 

 supplied at more than a certain rate. 



Fig. 5. 



It is well known that this tendency to superheating of the liquid is diminished 

 by stirring or shaking, but this leaves the liquid still in contact with the highly 

 heated bottom of the vessel. Any method which would keep the liquid mixed 

 and at the same time prevent the heating of the containing vessel to a temperature 

 greatly in excess of the boiling point of the liquid would provide almost ideal 

 conditions for rapid evaporation. 



Such a method is incorporated in the design of the evaporator described 

 below. In its simplest form this consists of a glass bulb containing the liquid, 

 and kept in constant slow rotation while it is heated by a gas burner. Owing to 

 the adhesion of the water to the glass, it is drawn up on the side of the bulb 

 which is rising, and a continuous thin film is spread over the upper part of the 

 bulb. This ensures very thorough mixing of the liquid, and also materially 

 increases the surface of liquid from which free evaporation can take place. At 



