714 
Proceedings of the Royal Society 
extreme state of division was not large enough for me to get it off 
the filter and examine it further ; hut after the treatment which 
it had received — it still remaining a brownish-black powder — there 
can he no doubt of its being carbon. 
A similar case of diffusion takes place on a small scale when we 
hold a cold porcelain lid over a bunsen flame, when, as is well- 
known, we obtain a black deposit under the glaze of the porcelain 
without the latter being fused. Here the carbon in the impalpable 
condition diffuses itself into the porcelain, hut aided by the convec- 
tion currents of the gases of the lamp. 
5. On the Variation with Temperature of the Electric Eesist- 
ance of certain Alloys. By Professor J. G. MacGregor 
and C. G. Knott, D.Sc. 
6. Preliminary Eeport on the Tunicata of the “ Challenger” 
Expedition. Part II. By W. A. Herdman, D.Sc. 
{By permission of the Lords Commissioners of the Treasury.) 
Since the publication of the first part of this preliminary report 
(Proc. Eoy. Soc. Edin., 1879-80, p. 458), I have received some 
additional specimens of Ascidians belonging to the “ Challenger ” 
collection, and including the following Ascidiada;. 
Ascidia cylindracea , n. sp. 
External appearance. — Shape nearly cylindrical; posterior end 
rounded and wider than truncated anterior end; ventral edge nearly 
straight, dorsal slightly concave. Attached by base and lower half of 
left side. Both apertures at anterior end; branchial towards ventral 
side, sessile; atrial on dorsal edge, forming a rounded projection; 
both distinctly lobed. Surface smooth. Colour yellowish -grey. 
Length, 2 cm. ; breadth, 1*2 cm. 
Test of moderate thickness, transparent, showing vascular rami- 
fications. 
Mantle having well-marked muscular bands. 
Branchial sac extremely delicate ; vessels very slender. Stigmata 
long and narrow, some being twice as long as others in consequence 
