SCIENTIFIC SUMMAKY. 
313 
It consisted] of three broad bands, the brightest parts of which had the 
following wave-lengths : — 
No. 1 
2 
3 
m.m.m. 
547-6 
515-6 
469-6 
Nos. 2 and 3 were sharply bounded on the less refrangible side, fading off 
gradually towards the violet. No. 1 was very ill-defined on both sides, and, 
being without any definite brighter part, its wave-length is very uncertain. 
Observers, Ralph Copeland and J. G. Lohse. 
Phenomena for the Quarter. — The sun will be eclipsed, but not visibly in 
England, on July 18. Yenus will be at her greatest brilliancy, as an evening 
star, on August 19 ; and in inferior conjunction with the sun on September 
24. Mercury will be in inferior injunction on August 24. Jupiter will be 
in opposition to the sun on August 31. Saturn (this however belongs to the 
following quarter) on October 5. 
BOTANY. 
The Black Mildew of Walls. — In “ Science Gossip ” for August last there 
was an article by Professor Paley, entitled “ Is the Blackness on St. Paul’s 
merely the Effect of Smoke,” in which the author maintained that this 
blackness is chiefly due to the growth of an undescribed lichen, which appears 
to flourish only on limestone and in situations unaffected by the direct rays 
of the sun. Professor Leidy, calling the attention of the Academy of 
Natural Sciences of Philadelphia to this paper (September 3, 1878), 
remarked that he had himself many years ago noted a similar black appear- 
ance on the brick walls and granite work of houses in narrow shaded streets, 
especially near the Delaware River. Noticing a similar blackness on the 
bricks above the windows of a brewery, from which there was a constant 
escape of watery vapour, he was led to suspect that it was of vegetable 
nature. On examination it proved to be caused by an Alga, closely allied to 
what he regarded as Protococcus viridis , which gives the bright green colour 
to the trunks of trees, fences, and walls, usually on the shady side. It may 
be the same plant in a different state, but until this is proved he proposes to 
name it Protococcus lugubris. It consists of minute round or oval cells, 
from 0-006 to 0*009 millim. in diameter, isolated, or in pairs or groups of 
four, the result of division, or in short irregular chains of from four to a 
dozen, sometimes with a lateral offshoot of two or more cells. 
Irritable or Sensitive Sta?nens . — In Part III. of the 11 Proceedings of the 
Academy of Natural Sciences of Philadelphia ” for 1878, Mr. Thomas Meehan 
calls attention to the large number of plants now known which exhibit an 
irritative motion in some of their parts, and states that from his observations 
of plants of the orders Bignoniacece, Sci'ophvlariacece , and Acanthacece , he 
has arrived at the conclusion that wherever there are bilobed flattened 
stigmas in these orders this sensitiveness to touch would be exhibited in a 
greater or less degree. He then remarks upon the fact that though in the 
Opuntia, a genus of Cactacece, the stamens move in various directions when 
