Prof. TyndalFs Contributions to Molecular Physics, 441 



an inch thick. Into it, at its corners, are rigidly fixed four 

 upright pillars, furnished at the top with screws, for the recep- 

 tion of the nuts qrst. D E F is a second plate of brass of the 

 same size as the former, and pierced with holes at its four cor- 

 ners, so as to enable it to slip over the four columns of the plate 

 A B C. Both these plates are perforated by circular apertures, 

 mn and op, 1*35 inch in diameter. GHI is a third plate of 

 brass of the same area as D E F, and, like it, having its centre 

 and its corners perforated. The plate G H I is intended to sepa- 

 rate the two plates of rock-salt, which are to form the walls of 

 the ceil, and its thickness determines that of the liquid layer. 

 Thus when the plates ABC and DEF are in position, a space 

 of the form of a shallow cylinder is enclosed between them, and 

 this space can be filled with any liquid through the orifice k. 

 The separating plate GHI was ground with the utmost accu- 

 racy, 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 un- 

 screwed, 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 GHI. 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 ap- 

 plied 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 experi- 

 ment, the apparatus 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- 



Phil Mag. S. 4. Vol. 28. No. 191. Dec. 1864. 2 G 



