Properties of the Alloys of Iron and Aluminium. 131 



the double purpose of protecting them from the action of the 

 furnace gases and at the same time of maintaining the tempe- 

 rature round them comparatively steady. 



The arrangement of the apparatus is shown in fig. 5. 



Ffc. 5. 



M 



VBBBZZZBZZZZZZ& 



JL 



M = muffle. 

 A = air-space. 

 C = copper box. 

 T = copper tube. 



B, B = asbestos board. 

 L = primary leads. 

 I = secondary and com- 

 pensating leads. 



A small governor was connected up between the main 

 gas-supply in the laboratory and the furnace. By this means 

 the pressure of the gas at the furnace was kept constant. 



The insulation-resistance between the primary and secondary 

 was tested from time to time during the experiments. It was 

 found to be always greater than 500,000 ohms. 



(2) Experiments at Temperatures between 0° C. and 50° C. 



The ring was placed in a thin- walled copper box, which it 

 almost completely filled. A piece of copper tubing of about 

 one inch in length was fastened to one side of the box, and to 

 this was attached a piece of glass tubing of about one foot in 

 length, through which the leads passed. The glass tube was 

 attached to the brass one by means of a joint made with 

 Thomas's alloy. This alloy melts at about 75° C. and expands 

 considerably on solidifying. If a piece of glass tubing is 

 placed concentrically within a brass tube of somewhat greater 

 diameter, and the melted alloy is run into the clear space 

 between the two tubes, a very good mechanical air-tight joint 

 is obtained. A glass tube is used to carry the leads in pre- 

 ference to a metal one, as it is desirable to prevent, as far as 

 possible, the passage of heat between the cavity containing 

 the ring and the external air. 



In working at temperatures much below zero this pre- 

 caution was found very necessary. 



The general arrangement of the apparatus is shown in 



K2 



