Heat and Light from Heated Solid Bodies. 563 



flask of phosphorus pentoxide attached to the pump by a wide 

 side tube, and so arranged that it does not choke the connexion 

 between the pump and the apparatus ; (2) four pockets, two 

 on each experimenting tube, as shown in sketch, each one 

 containing phosphorus pentoxide. These pockets are short 

 side tubes of the same diameter as the main tubes, and they 

 allow free access for diffusion of the water-vapour into the 

 drying material. 



This method of arranging the drying-tubes seems to me to 

 be far better than the usual plan of putting a good long 

 column of drying material between a pump and the apparatus 

 to be exhausted. The effect of the latter is only to dry the 

 gas that is pumped away, not to dry the gas remaining in the 

 experimental apparatus, except by diffusion. Moreover, such 

 an arrangement must very seriously retard the process of 

 exhaustion at very low pressures, by choking the tube. 



The vacuum is measured by a modification of the McLeod 

 gauge attached to the second of the end tubes mentioned 

 above (see PI. V.), so that the experimenting tubes are between 

 the pump and the gauge. By this arrangement it may be 

 safely assumed that the vacuum in the experimenting tubes 

 is at least as good as the vacuum in the gauge, especially as 

 the gauge W as so made as to allow a wide tube connexion 

 with the apparatus, in order to assist diffusion and prevent its 

 readings from lagging behind w T hat they should be. 



The electrical connexions, by means of which the currents of 

 electricity, which heat the strips, are passed into them and are 

 measured, will be easily understood from the figures (PL V.). 

 The current is supplied by a storage-battery of ten cells. 

 There are two separate circuits, one for each strip. The 

 current sent into each is regulated partly by the number of 

 cells, and partly by a rheostat in circuit; and an amperemeter 

 in each circuit measures the current passing. 



A voltmeter can be applied to the working part of either 

 strip by means of the potential electrodes, EE, E'E', and the 

 double-pole change-over switch indicated in the figure. 



It will be seen from the elevation on PL V. that the 

 electrodes, EE, are attached at some distance from the ends 

 of the strip AB. This is to avoid disturbance from the cooling 

 effect of the end attachments. It is the portion EE which I 

 call the working part of the strip. 



In a very interesting paper by W. H. Weber, published 

 in the Annalen der Pliysik und Chemie, vol. xxxii. p. 256, a 

 paper which I think has scarcely received the attention 

 which it deserves, the author traces from the very commence- 

 ment the production of light by a heated solid body. Pie 



