234 Dr. W. Marcet. 



carried away by a stream of water ; according to the authors, tests 

 showed this calorimeter to be very accurate. 



The construction of the present instrument was suggested from 

 Berth elot's calorimeter, for the determination of specific heat by 

 mixtures, in which the heat given out is reflected by bright surfaces 

 of silvered copper surrounding at a short distance the vessel con- 

 taining the mixture. It was found, however, in experimenting with 

 the new calorimeter (which was not silvered inside), that all the 

 heat was not reflected, a certain proportion being absorbed by the 

 copper, which necessitated an arrangement for determining the 

 temperature of the metal. 



The instrument (see figure) consists of a wooden chamber lined 

 internally with a thick padding of cotton wool, and externally with 

 several thicknesses of felt. Inside this chamber there is another 

 made entirely of sheet copper, the inner surface of which is main- 

 tained carefully polished ; its height is 145 cm., and its breadth is 

 69 cm. The two chambers have between them an annular space 

 from 4 to 5 cm. in breadth. The capacity of the copper chamber, 

 empty, is 81O4 litres, and its weight 62'370 kilograms, therefore an 

 alteration of 1 C. in the temperature of that mass of copper would 

 be equal to 5832 (small) calories, or would raise 5832 grams of 

 water 1 C. 



The copper chamber is closed by means of a movable panel also 

 constructed of copper, which is fixed to a wooden backing ; the edge 

 of the copper panel is made to press tightly against an india-rubber 

 cushion carried round the rim of the opening in the copper chamber, 

 while the edge of the wooden backing is applied against the rim of 

 the wooden chamber, and the panel is kept in its place firmly with 

 brass screws. This movable door is too heavy to be handled easily 

 by one man, and on that account is fixed to a tackle fastened to a 

 beam in the roof of the laboratory, by which means no difficulty is- 

 experienced in opening or closing the chamber. 



There is a small window 21 x 15 cm., made of two superposed 

 panes of very thick glass, and opening into the two chambers ; when 

 closed, the rim of the inner pane presses against a cushion around 

 the corresponding opening in the copper chamber; it shuts by a 

 spring bolt, and should the person under experiment feel uncom- 

 fortable, or wish to communicate with the outside, he can push open 

 this window at any time. 



Inside the copper chamber there are two ventilators, or perhaps 

 more correctly " agitators," in the form of revolving fans, the object 

 of which is to thoroughly mix the air inside the chamber. One of 

 these agitators is fixed high up in the chamber, the other low down 

 on the opposite side. The upper agitator was found experimentally to 

 produce a blast of 190 litres per minute say about 380 litres per 



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