the Radiation of Heated Gases. 471 



iron tubes in place of porcelain. The first trials were made 

 with a short piece of iron tube about 6 cm. long and 8 mm. 

 bore, bevelled at the ends and fitted between two hard glass 

 tubes of the same diameter, the joints being ground to fit. 

 This made good joints when the iron became red-hot and the 

 glass in contact with it was softened and pressed up tight ; 

 but although satisfactory results were obtained in two or 

 three experiments, constant trouble was experienced in the 

 cracking of the glass while cooling. Finally the glass 

 was discarded and a long iron tube prepared (a piece of 

 ordinary ^-inch hydraulic tube). In order to diminish as far 

 as possible the loss of heat by conduction, so as to maintain 

 a high temperature for about 8 cm. in the central part, a 

 number of deep necks were cut in the metal, as shown in 

 fig. 2, and around these necks a thick ring of asbestos-packing 

 was wound and a fire-clay arch placed over all. Thus the 

 Bunsen flame could be concentrated entirely on the central 

 piece of the tube and the temperature could be maintained 

 inside the tube above the fusing-point of fluor-spar and 

 aluminium, but not reaching that of silver. The rubber 

 connexions between the ends of the iron tube and the two 

 T-pieces gave trouble at first, but subsequently it was found 

 that when buried in a thick layer of plaster of Paris they 

 were completely protected from destruction by heat, the large 

 surface afforded by the plaster of Paris forming an effectual 

 radiator and preventing the ends from becoming too hot. 



Experiment V. — With the iron tube at a bright red heat, 

 the sodium was tipped out from the steel spoon as before, 

 in an atmosphere of carefully dried coal-gas. Now, if in the 

 previous experiments the D radiation was due to chemical 

 action taking place between the sodium and the oxygen 

 compounds of the porcelain, one ought in this experiment to 

 find, if not an entire suppression of the bright line, at least 

 a striking difference in the radiation. No such difference 

 was, however, to be observed, the intensity remaining pre- 

 cisely the same as before and the sequence of phenomena 

 closely resembling that shown in fig. 3. The various phases 

 there shown were, however, prolonged almost indefinitely in 

 time, as the sodium never became used up as before, and the 

 distillation into cooler parts of the tube proceeded so slowly 

 that it was necessary to allow a gentle current of gas to drive 

 away the denser vapour giving a continuous spectrum before 

 the D line itself could be studied. Also throughout the 

 experiment the dark absorption-line was more intense than 

 in porcelain, and it could easily be observed after six hours 

 of continuous heating, when even the emission-line had 

 become relatively narrow. This naturally follows from the 



