506 MR. J. B. PBTAVBL ON THE HEAT DISSIPATED BY 



The emissivity in steam at atmospheric pressure is given in fig. 5. The slope of 

 the curve is much sharper than for any of the gases previously studied. This fact, as 

 we shall see from the results given below, cannot be entirely accounted for by the 

 higher temperature of the enclosure. 



I hope shortly to undertake a research on the emissivity of platinum in gases at a 

 much higher pressure. For the present the results are too incomplete to allow any 

 general theory to be formulated. 



The values obtained for the emissivity are to some extent dependent on experi- 

 mental conditions, such as the diameter of the radiating wire, its position in sp;ice 

 (whether horizontal or vertical), the dimensions of the enclosure, its absolute tempera- 

 ture, and the material of which it is made. For the results to be of any general 

 application, it is necessary to form an idea of the extent to which they are affected 

 by these divers factors. 



Curve I. in fig. 7 (Plate 20) is taken with the radiating wire in the square gun-metal 

 box described in Part III. of this work. For Curves II. and III. the wire was placed 

 in the axis of an iron cylinder 2'6 centims. in diameter, 27'5 centims. long. In all 

 three cases the wire was horizontal. 



Curve IV. has already been given in fig. 2 ; it refers, a.s do all the results given 

 above, to a wire '112 centim. in diameter placed vertically in the axis of a glass 

 cylinder 5 '8 centims. in diameter. 



The difference between the Curves I. and IV. is mainly due to the change of posi- 

 tion. When the wire is vertical, the part on which the observations are taken is 

 surrounded by a layer of gas which rises from below after being heated to sub- 

 stantially the same temperature as the wire itself. When the wire is horizontal, the 

 hot gas rising from it is replaced by a fresh supply at nearly the same temperature as 

 the enclosure. The variation between Curves I. and II. is about 10 per cent. It is 

 a difference of this oi'der that we may expect when the shape, size, and material of 

 the enclosure are radically changed. In cases, however, where the ratio of the surface 

 of the radiator to the surface of the enclosure is not kept very small, the total loss of 

 heat depends very largely on the dimensions of the enclosure. 



Curve III. is taken in the same iron cylinder as used for Curve II. In this case 

 the iron was kept at a high temperature by surrounding it with a jacket containing 

 boiling sulphur. The temperature of the enclosure, as measured by the platinum 

 wire subsequently used as the radiator, was somewhat high. This temperature, 

 namely, 455 C., or ten degrees above the usual boiling point of sulphur, can easily 

 be accounted for by the direct heating effect of the flame, for the prevention of which 

 no precautions were taken. 



The slope of the curve of emissivity obtained at this temperature is considerably 

 steeper than that relating to experiments with the enclosure at a lower temperature. 



The above-mentioned curves, which are given in fig. 7, all refer to the emissivity 

 in dry air at atmospheric pressure. 



