640 Of the Excellent Qiialities of Coffee. 



ally on the under side with a brush or a towel, but it 

 should not be made bright ; for when it is bright it will 

 be more difficult to heat the water in it than when it is 

 tarnished and of a dark brown color. 



But the sides of the boiler should be kept as bright 

 as possible ; for, when its external surface is kept clean 

 and bright, the boiler will be less cooled by the sur- 

 rounding cold bodies than when its metallic splendour 

 is impaired by neglecting to clean it* 



As the small quantity of water which is put into the 

 boiler serves merely for generating the steam which is 

 necessary in order to keep the reservoir and its contents 

 constantly boiling hot, if the reservoir be made of sil- 

 ver or even of common tin, the boiler may without the 

 smallest danger be made of copper, or of copper 

 plated with silver, which will give to the boiler an ele- 

 gant appearance, and at the same time render it easy 

 to keep it clean on the outside. 



The boiler may likewise be made of tin, and neatly 



* I have in my possession two porcelain tea-pots of the same form and 

 dimensions, one of which is gilt all over on the outside, and might easily be 

 mistaken for a gold tea-pot ; the other is of its natural white colour, both within 

 and without, being neither painted nor gilt. When they are both filled at the 

 same time with boiling water, and exposed to cool in the same room, that which 

 is gilt retains its heat half as long again as that which is not gilt. The times 

 employed in cooling them a given number of degrees are as three to two. 



The result of this interesting experiment (which I first made about seven 

 years ago) affords a good and substantial reason for the preference which Eng- 

 lish ladies have always given to silver tea-pots. The details of this experiment 

 may be seen in a paper published in the Memoirs of the French National Insti- 

 tute for the year 1807. 



I have likewise a set of tea-cups and another of coffee-cups, which are gilt 

 on the outside, and they preserve the heat of those liquids much longer than 

 China cups which are not so gilt. 



Little advantage would be derived from gilding them on the inside, and none 

 at all if they were filled quite full with the hot liquid. 



I have found that all metals are alike useful in preserving heat (or cold), pro- 

 vided their surfaces be quite clean and bright. 



