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-hlack 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, but 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 cylindraeea , 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 
