THE PRESIDENT'S ADDRESS. 
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solid gelatine, are groups of bacilli, arranged in various forms. 
Besides groups which are in direct continuity with the track, 
there are others scattered about, apparently unconnected with the 
main mass. These vary in shape from the incessant movement 
of the bacilli. In plate cultivations the colonies are, as a rule, 
at first globular; but very soon the bacilli are seen to shoot 
into the gelatine, forming loops, and these loops often run out 
for a long distance into the jelly, and are twisted, giving rise to 
what Hanser terms " spiraline forms." 
Bacteria may be divided into four groups, according to the 
effects produced in the media in which they grow. 
I. The choromogenic or pigment-forming variety. This species 
may be said to be of historical interest. In the Book of Exodus 
it is said, that "the Lord spake unto Moses, Say unto Aaron, 
Take thy rod, and stretch out thine hand upon the waters of 
Egypt, upon their streams, upon their rivers, and upon their ponds, 
and upon all their pools of water, that they may become blood ; 
and that there may be blood throughout all the land of Egypt, 
both in vessels of wood, and in vessels of stone. And Moses and 
Aaron did so, as the Lord commanded ; and he lifted up the rod, 
and smote the waters that were in the river, in the sight of 
Pharaoh, and in the sight of his servants ; and all the waters 
that were in the river were turned to blood. And the fish that 
was in the river died; and the river stank, and the Egyptians 
could not drink of the water of the river ; and there was blood 
throughout all the land of Egypt." This variety occurs in marshy 
districts, forming a blood-red coloured growth on the surface of 
pools and lakes, and it is probably in them we find an explanation 
of the plague above referred to. A similar growth is sometimes 
found in snow. It is known by the name of micrococcus pro- 
digiosus. It has also been found on bread, and in an epidemic form 
attacked the bakehouses in Paris. It has appeared spontaneously 
on the sacred bread or host, and has led to pilgrimages to see it. 
Another growth produces a blue colour in milk, not the sky- 
blue which we sometimes hear of. There are other chromogenic 
bacteria, which produce a violet pigment, and one a delicate 
green. Another micrococcus recently isolated is called magenta 
micrococcus, and in this species the pigment is retained in the cells. 
II. The zymogenic bacteria are those which produce fermenta- 
tion ; such as in the fermentation of malt, one variety tending 
