386 Royal Society :-— 



served that silver lines remained in the alloy as long as from *05 

 to '02 per cent, of silver was present. The alloys, however, were 

 very unequal. Experiments on cadmium and tin alloys are de- 

 scribed, the cadmium forming 10, 5, 1*0, 0*15 per cent. In the 

 last but one cadmium line was permanent ; in the first at least five 

 were seen. In an alloy of 0*099 per cent, of cadmium with a mix- 

 ture of lead, tin, and zinc constituting the rest of the alloy, the 

 behaviour of the cadmium lines was sensibly the same as in a mix- 

 ture of 0*1 per cent, of cadmium and 99*9 of tin. 



In the Mint specimens the same phenomena were observed m 

 petit as the coarser alloys showed en grand. In a gold-copper alloy 

 yjy 0- increase in the gold made the lines shorter, and a similar in- 

 crease in the copper made them longer. 



In the silver-copper alloy an increase of yxfro* ^ n ^ ne SUYer 

 lengthened the lines, a similar increase in the copper shortened 

 them. 



These phenomena can be explained by assuming such alloys 

 to be different physical things, and that the spark acts upon the 

 alloy as a whole as well as upon each vapour separately. 



Thus, in these Mint alloys, copper is common to both, and their 

 melting-points are : — 



Gold.... 1200° (Pouillet). 



Copper . . 1200° to 1000°, the precise point not determined. 



Silver .. 1000° (Pouillet). •' 



The intermediate position of copper explains the different action 

 on its lines of gold and silver. 



II. The Method of photographing Spectra adopted. 



A camera carrying a 5 x 5-inch plate and a 3-inch lens of 23 inches 

 focus, replaced the observing-telescope of the spectroscope. The 

 lens focused from 3900 to 4500 very fairly upon the plate. The 

 beam passing through collimator and prisms was, as in Mr. Euther- 

 ford's researches, very small. As the electric arc in its usual ver- 

 tical position gave all the lines from pole to pole, the lamp was 

 placed on its side, and the arc used in a horizontal position, the slit 

 being vertical. The dense core of the arc then gave all the short 

 lines in the centre of the field, the longer ones extending beyond 

 them on either side. In order to obtain a scale, it was resolved 

 to photograph the solar spectrum immediately adjacent to the 

 metallic spectrum under examination. 



To effect this a portion of the slit was covered up while the 

 solar spectrum passed through the free part, and then the part 

 used for the solar spectrum was covered, while the formerly covered 

 part was opened for the metallic spectrum. This was effected 

 by a shutter, with an opening sliding in front of the slit ; a dia- 

 gram of its action and form is given. 



The arrangement of the spectroscope, heliostat, <fec. for obtain- 

 ing the sun's light is described. The image of the sun was brought 



