198 ANNUAL, RTCPORT SMITHSONIAN INSTITUTION, 1918. 



The most striking difference between the new black phosphorus 

 and previously known modifications is its high density. The density 

 of ordinary solid white phosphorus is 1.83, and that of red phos- 

 phorus may vary according to the method of preparation from 2.05 

 to a maximum of 2.34 for Hittorff's "metallic" crystallized red 

 phosphorus. Nine determinations of the density of different speci- 

 mens of black phosphorus were made. We accept the value 2.691 

 as the true density of black phosphorus, a value 15 per cent higher 

 than that of the most dense variety of red phosphorus. The con- 

 clusion is inescapable that this is a new modification of phosphorus, 

 quite distinct from red phosphorus, and because of its higher density 

 presumably a more stable form. 



Black phosphorus does not catch fire spontaneously, can be ignited 

 with difficulty with a match, and may be heated to perhaps 400° in 

 the air without spontaneous ignition. Unlike commercial red phos- 

 phorus, it can not be ignited by striking with a hammer on an anvil. 

 It is almost, if not entirely, stable in the air. A few simple tests 

 seemed to show that it is much like red phosphorus in chemical 

 properties; it is attacked by cold nitric acid, is not acted on appre- 

 ciably by sulphuric acid, and is not dissolved by carbon disulphide. 



When black phosphorus is heated in a closed glass tube it vapor- 

 izes and condenses in the colder parts of the tube to red and white 

 phosphorus. The appearance under these conditions is exactly 

 the same as when red phosphorus is similarly treated. It would 

 seem, therefore, that the vapors of black and red phosphorus are, at 

 least in large part, identical. 



Black phosphorus is a fairly good conductor of electricity, in dis- 

 tinction from white and red phosphorus, which in the pure state 

 seem to be nearly perfect insulators. The specimen of black phos- 

 phorus whose conductivity was measured here was selected from 

 all the available pieces for its great apparent compactness. It Avas 

 prepared by turning in a lathe, leaving for the final test a cylindrical 

 piece about 1.52 centimeters in diameter and 2.69 centimeters long. 

 The electrodes were attached by copper plating terminals on the 

 plane ends and soldering copper wires to the copper plating. 



The value found for the specific resistance is 0.711 ohms per cen- 

 timeter cube at 0°. The temperature coefficient of resistance has a 

 large negative value, and between 0° and 75° the relation between 

 temperature and resistance is nearly linear. At 0° it is —0.00465. 

 This is an unusually high value, higher than for any substance usu- 

 ally listed. It is about 10 times higher than for carbon, and makes 

 it practically certain that the small amount of carbon known to be 

 present can not be taking a large share in the conduction. It should 

 be remarked that in respect to the sign of the temperature coefficient 

 black phosphorus is not like the metals. 



