Ward. — A Violet Bacillus from the Thames. 61 
surfaces of these colonies appear granulated and rugose, as if 
marked with a delicate and complicated series of contours 
and mounds, giving a characteristic aspect, which, however, is 
by no means peculiar to this specific form — e. g. such colonies 
occur in Typhoid (Fig. 5). 
In four or five days the colonies appear as milk-white, 
opaque, thin, discoid, smooth and glistening expansions to 
the unaided eye, though the microscope shows the same 
characters as described above. They grow very slowly, and 
even after ten days at 20° C. may be still white, though 
somewhat thicker, like an opaque, flattened milky drop, about 
4 to 5 mm. in diameter, and slightly sinking in the now 
softening gelatine. They are, in fact, beginning to liquefy 
the latter. 
About this period, also, the superficial colonies are usually 
beginning to show the violet pigment — though sometimes it 
occurs much earlier and at others is delayed — the colour 
appearing first as a mere trace of purplish cast on a dirty- 
white matrix, gradually spreading over the colony, till the 
latter is deep violet or almost blue black. The pigment does 
not spread into the gelatine, but is strictly confined to these 
superficial colonies, at least for a long time. The totally 
submerged colonies may remain dirty-yellow, or buff-coloured, 
even for weeks, so that oxygen may have to do with the 
development of the pigment ; in some cases, however, I have 
found them faintly purple in four or five days (cf. Figs. 4 -6). 
It is interesting to note that slow growth, with the 
formation of the pigment, occurs at temperatures even as low 
as 5 0 or 6° C. on gelatine-plates kept cool by ice. 
If there are very few colonies on the plate, they may 
eventually dry up with the non-liquefied gelatine, and I have 
plates in the laboratory more than two years old where this 
has occurred. The hard, smooth, deep purple discoid colonies 
on these plates look just like dried ink-drops, each about 
10-12 mm. in diameter, with irregular margins, and each 
surrounded with a pale zone, 2-6 mm. broad, scooped out of 
the yellower dry gelatine, and representing where liquefaction 
