ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 653 



tion, for example, of a Grlans prism (involving an air-layer) it is of great 

 advantage to avoid heating, because the cement is apt to fly into the 

 air-layer. In such a case a lateral plane should be polished on to the 

 nicol ; the glass prism is not necessary because the angle a is less than 

 the angle of total reflexion. 



Allotropic Conversion of Phosphorus in the Cardioid-Ultramicro- 

 scope.* — H. Siedentopf describes how the actual conversion of white 

 phosphorus into its allotrope, red phosphorus, can be watched. For this 

 purpose it is necessary to introduce a small piece of the white variety 

 into the quartz -chamber of the cardioid-ultramicroscope and to use a 

 magnification of 1500 diameters. The agent of conversion is the light 

 of the visible spectrum. The long-waved rays are kept off by long 

 water chambers and the short-waved are absorbed through the glass of 

 the illuminating lenses. The sharply defined circular field of the car- 

 dioid-condenser is controlled by the image of the illuminating lens 

 projected through the condenser and only a very small field at first 

 obtained. An arc-lamp is the light-source. Almost immediately after 

 the admission of light into parts which were previously optically empty, 

 there appear white sub-microscopic specks at a distance apart of perhaps 

 half a micron. The brightness of these specks (? luminiscence) increases 

 rapidly, so that further observation on account of excessive brightness is 

 impossible. If a cobalt glass disk, which cuts off the yellow rays and 

 transmits the red and blue, be laid on the ocular, the change can be 

 further followed and the complete conversion into red seen. The first 

 step in this conversion seems to be a colloidal phase. Before using the 

 cobalt glass place a matt disk immediately under the condenser. Gradually 

 larger and less approximate particles make their appearance. They con- 

 tinue to increase in brightness, but are uo longer round ; they emit whitish 

 prolongations, partly rectilinear, partly slightly curved, on three or more 

 sides. These prolongations are unpolarized, so that nothing can be said 

 as to a possible crystalline nature. If the matt disk be now removed 

 and strong light allowed to enter freely, these particles with their pro- 

 longations develop in a few seconds an intensive light-emission ; with the 

 cobalt-glass disk their red tint may be recognized. More central particles 

 and prolongations reveal themselves, and gradually the whole field is 

 filled'with the gleam of a reddish meshwork. This new state of the 

 phosphorus remains permanent in darkness. Similar demonstrations 

 may be made successively of different parts of the field. 



If solution of phosphorus in carbon disnlphide be used, instead of 

 pure phosphorus, the effects are similar. There is, however, also an 

 earlier phase. If the light be darted, lightning-like, into areas which 

 were previously optically empty, white sub-microns are seen in active 

 molecular movement ; but they are quickly absorbed, and remain clinging 

 to the walls of the quartz -chamber. In gold solutions this molecular 

 movement may last three days before absorption. The author has ex- 

 perimented with arsenic and selenium with a similar series of results. 

 He has also observed the reduction of potassium bichromate, which under 

 the influence of light takes place in a few seconds. 



* Ber. Deutsch. Chem. Gesell., xxxiii. (1910) pp. 692-4. 



