330 Lord Blythswood. On the Reflection of Rontg en [Mar. 19, 



supposition that, as suggested by Lockyer and by Runge and Paschen, 

 the helium from various sources is not quite homogeneous, but that 

 different samples differ slightly in density. I think that these num- 

 bers point to a possible division into groups. The gas from brog- 

 gerite appears to have the density 2*18, that from samarskite 2*12, 

 and that from fergusonite 2*14. But the evidence is slender. It is 

 not impossible that the gas from cleveite is lighter ; it is unfortunate 

 that the sample of cleveite gas was lost ; but its mixture with that 

 from broggerite weighs less than the gas from broggerite alone. 



It has not, I think, been noticed that the light emitted from a 

 vacuum-tube containing cleveite gas has a richer orange-yellow shade 

 than is shown by gas from broggerite, samarskite, and fergusonite. 

 This is doubtless due to a greater intensity of the red line ; but there 

 is another difference. The cleveite gas shows, in addition to the 

 usual strong lines, a set of fairly strong lines between the very strong 

 green, and the strong blue. These lines have never been observed in 

 samples of gas from broggerite, samarskite, or fergusonite, no matter 

 how high or low the pressure in the tube, or the intensity of the dis- 

 charge. But the difference of colour is so marked that it is easy, at 

 a glance, to say whether any tube contains cleveite-helium, or helium 

 from another mineral. 



Taking the two lines of evidence together, they undoubtedly 

 strengthen each other, and I am at present engaged in an attempt to 

 fractionate the mixed gases from broggerite, samarskite, and ferguso- 

 nite into two or more portions. 



Mention should perhaps be made, in conclusion, of Langlet's deter- 

 mination of the density of cleveite-helium (' Zeit. anorg. Chem.,' 10, 

 287). The number he obtained, weighing in a globe of 100 c.c. 

 capacity, was 2 '00. His sample was examined spectroscopically, and 

 appears to have been free from nitrogen and hydrogen. It is unlikely 

 that he can have made an error of 1*3 milligrams in weighing, 

 especially as he determined the density three separate times. It is 

 not impossible, therefore, that he had under his hands gas much 

 richer in some light constituent than it has been my fortune to fall 

 in with. Whether this is the case or not, further research must 

 show. 



III. " On the Reflection of Rontgen Light from Polished 

 Speculum-Metal Mirrors." By Lord Blythswood. Com- 

 municated by Lord Kelvin, F.R.S. Received March 13, 

 1896. 



It has been generally supposed that the X rays cannot be re- 

 flected. 



