neighbourhood of strongly Illuminated Bodies. 225 



magnesic- oxide smoke. The light revealed little growing 

 bushes or trees of aggregating magnesic-oxide particles. The 

 mica was evidently electrical. Rubbed sealing-wax acted in a 

 similar way, but so did an unrubbed rhomb of Iceland spar to 

 a slight extent. We at first thought that the mica might be 

 photo-electric like certain crystals observed by Hankel. All 

 pieces of mica do not behave in this way. Pressure between 

 the finger and thumb, writing upon the surface with a blunt 

 point of cork, or with a pencil on a piece of paper laid upon 

 the mica, all seem capable of producing the deposition of the 

 white dust upon these portions, but in a lesser degree. There 

 would seem to be a possible relation between these effects 

 and the " Hauchbilder " of Moser (Wiillner, Physik, Bd. i. 

 S. 416, Dritte Auflage). Perhaps the removal of moisture- 

 condensing nuclei may not be without influence upon the 

 correctness of his explanation. The action in the case of 

 mica would not, however, seem to be wholly, nor neces- 

 sarily, electrical, as the following observation would tend to 

 show. A Chladni brass plate was casually used by the assis- 

 tant as a desk upon which to support a piece of paper upon 

 which he was writing with a pencil. Upon subsequently 

 bowing the plate, after dusting Lycopodium upon it, the 

 writing became legible upon the surface of the brass. This 

 action of mica is not, so far as we have observed, produced 

 by simple heating. With a slab of tourmaline cut perpen- 

 dicular to the axis from a crystal belonging to Prof. N. Story 

 Maskelyne, the effects of heating, cooling, and re-heating 

 this pyroelectric crystal by the beam of light were very dis- 

 tinctly shown. We believe that some such simple and delicate 

 method as that here indicated may be of use in the investi- 

 gation and demonstration of such actions. 



4. Experiments made to ascertain the Effect of Pressure 

 in the Smoky Medium. 



The apparatus for this purpose has already been referred to ; 

 it is shown in fig. 9. It could be connected with a water 

 air-pump, which reduced the pressure to a few centimetres of 

 mercury, or with a large iron cylinder, in which the air was 

 condensed by pressure of the water-main. The open mercury 

 manometer indicated a pressure of 4^ atmospheres (in excess 

 of the atmospheric pressure). 



Tobacco smoke was introduced by slight exhaustion, and 

 the small rod of electric-light carbon, shown in section in 

 fig. 9, was heated by the electric-light beam being con- 

 verged upon it. Under an absolute pressure of 5^ atmo- 

 spheres the convection-currents became very slow, the black 



Phil Mag. S. 5. Vol. 17. No. 105. March 1884. Q 



