188 Domestic Science 



bell- jar, with a tightly-fitting rubber stopper, through 

 which pass two glass tubes as shown. The flask below 

 is likewise fitted and is completely filled with water. 

 Pour enough water into the bell-jar to cover the end of 

 the topmost tube. Drop a few crystals of potassium 

 permanganate (or Condy's crystals will serve equally 

 well) into the liquid in the jar. Fix the apparatus 

 in an upright position by means of a clamp and retort 

 stand, at such a height that a Bunsen flame may be 

 placed underneath the flask. Light the gas and gently 

 heat the flask with a small flame. The water will 

 pass along the straight tube into the bell- jar and a 

 descending current will flow down the bent tubing 

 to the bottom of the flask. The course of the liquid 

 may be traced by the pink colour of the solution 

 of potassium permanganate. The circulation can be 

 watched until the whole of the water has assumed a 

 uniform tint. 



The flask represents the boiler situated in the 

 basement of a building, the vertical tube corresponds 

 to the iron pipe leading from the top of the boiler 

 to the top of the main hot cistern, the latter is repre- 

 sented by our bell-jar and the bent tube is comparable 

 to the feed-pipes and radiators used to distribute the 

 hot water to the various floors of the building. The 

 smaller apparatus by which a supply of hot water 

 for the bath-room and for general domestic use is 

 maintained in most modern houses works on an exactly 

 similar plan. Further details on this head will be 

 found in Part II. 



125. The currents set up in a liquid as a result 

 of heating are called "convection currents", the heat 

 being said to be transmitted by convection. This 

 method of transmission does not take place with solids, 



