PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
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G H I was ground with the utmost accuracy, and the surfaces of the plates of salt were 
polished with extreme care, with a view to rendering the contact between the salt and 
the brass water-tight. In practice, however, it was found necessary to introduce washers 
of thin letter-paper between the plates of salt and the separating plate. 
In arranging the cell for experiment, the nuts qrst are unscrewed, and a washer of 
india-rubber is first placed on ABC. On this washer is placed one of the plates of 
rock-salt. On the plate of rock-salt is placed the washer of letter-paper, and on this 
again the separating plate G H I. A second washer of paper is placed on this plate, 
then comes the second plate of salt, on which another india-rubber washer is laid. 
The plate D E F is finally slipped over the columns, and the whole arrangement is 
tightly screwed together by the nuts qrst. The use of the india-rubber washers is 
to relieve the crushing pressure which would be applied to the plates of salt if they 
were in actual contact with the brass plates ; and the use of the paper washers is, as 
already explained, to render the cell liquid-tight. After each experiment, the appa- 
ratus is unscrewed, the plates of salt are removed and thoroughly cleansed ; the cell is 
then remounted, and in two or three minutes all is ready for a new experiment. 
My next necessity was a perfectly steady source of heat, of sufficient intensity to 
penetrate the most absorbent of the liquids to be subjected to examination. This was 
found in a spiral of platinum wire, rendered incandescent by an electric current. The 
frequent use of this source of heat led me to construct the lamp shown in fig. 2. A 
is a globe of glass 3 inches in diameter, fixed upon a stand, which can be raised and 
lowered. At the top of the globe is a tubulure, into which a cork is fitted, and through 
the cork pass two wires whose ends are united by the platinum spiral S. The wires 
are carried down to the binding screws a b, which are fixed in the foot of the stand 
2 y 2 
