76 Hon. R. J. Struit on the Absorption of 



(6) Given two angles and a side not included. 



Thi.s may be solved by applying (5) to the polar triangle^ 

 or we may proceed thus. Trace through the point P coin- 

 ciding with A, and that one of the circles m which makes an 

 angle with ADB equal to one of the given angles. On this 

 arc take PR equal to the given side. Turn the cloth in the 

 counter-clockwise direction until R lies on that one of the 

 lines m which makes an angle with ACB equal to the other 

 given angle. Isow trace through Q coinciding with A and 

 the arc PQ, and measure the triangle PQR. 



A net is not absolutely essential for many of the con- 

 structions, but then it is necessary to draw that portion of 

 the net which is needed for the question under discussion. 

 Many of the above problems could be solved still more rapidly 

 by the use of two nets of the same size, one of which is drawn 

 on tracing-cloth or some other transparent material_, and is 

 pivoted so that it can turn freely about the common centre of 

 the nets. This is the principle of Saxby^s spherograph. 



1 Bryn Don, Garth^ Bang-or, 

 North Wales. 



X. On the Absorption of Light by Mercury and its Vapour. 

 By the Hon. R. J. Steutt, Fellow of Trinity College, 

 Canibridge'^ . 



IN a recent paper f I drew attention to the remarkable 

 difference between the electrical properties of liquid 

 mercury and of mercury A'apour. It was found that at very 

 high temperatures and pressures the vapour showed relatively 

 considerable conductivity, and also distinct selective absorp- 

 tion of light. In the present paper I shall describe some 

 rough experiments to compare the optical transparency of 

 mercury vapour at the boiling-point of mercury with that of 

 the solid (frozen) metal. 



An iron gas-pipe, 3 metres in length, 2 cms. internal 

 diameter, was closed at each end with mica diaphragms. 

 The windows were fitted so as to be fairly air-tight by 



* Communicated by the Author. 



t Phil. Mag. XoT. 1902. I may take this opportunity of recording 

 an attempt to observe the critical phenomena of cadmium, on the same 

 lines as these described in that paper for mercury and arsenic. A piece 

 of clean cadmium Avas sealed up in a quartz tube, and the temperature 

 raised until the tube began to yield viscously. Nothing could be seen in 

 the part of the tube containing cadmiuia vapour. The vapour was quite 

 colourless. When the tube softened, the vapour blew a hole in the tube 

 wall, and escaped, oxidizing to a cloud of brown smoke. I cannot con- 

 firm the statement made in some chemical works to the effect that the 

 vapour of metallic cadmium is brown. 



