active, but ultimately cease, the pseudopodia-like processes being all retracted and the whole forming a motionless sphere.

Let the specimen cool again; the movements are not resumed; the protoplasm having passed into a state of permanent coagulation or rigidity.

7. Repeat the above observations on the white bloodcorpuscles of the frog or newt.

IV.

BACTERIA.

UNDER the general title of Bacterium a considerable variety of organisms, for the most part of extreme mínuteness, are included.

They may be defined as globular, oblong, rod-like or spirally coiled masses of protoplasmic matter enclosed in a more or less distinct structureless substance, devoid of chlorophyll and multiplying by transverse division. The smallest are not more than goth of an inch in diameter, so that under the best microscopes they appear as little more than mere specks, and even the largest have a thickness of little more than Tooth of an inch, though they may be very long in proportion. Many of them have, like Protococcus, two conditions-a still and an active state. In their still condition, however, they very generally exhibit that Brownian movement which is common to almost all very finely divided solids suspended in a fluid. But this motion is merely oscillatory, and is readily distinguishable from the rapid translation from place to place which is effected by the really active Bacteria.

In one of the largest forms, Spirillum volutans, it has been possible to observe the cilia by which the movement is effected. In this there is a cilium at each end of the spirally coiled body. No such structure, however, can be made out in the straight Bacteria, and it remains doubtful whether they possess cilia which are too fine to be rendered

visible by our microscopes, or whether their movements are due to some other cause. Many forms, such as the Vibriones, so common in putrefying matters, appear obviously to have a wriggling or serpentiform motion, but this is an optical illusion. In this Bacterium, as in all others, the body does not rapidly change its form; but its joints are bent zig-zagwise, and the rotation of the zig-zag upon its axis, as it moves, gives rise to the appearance of undulatory contraction. A cork-screw turned round, while its point rests against the finger, gives rise to just the same appearance.

Bacteria, in the still state, very often become surrounded by a gelatinous matter, which seems to be thrown out by their protoplasmic bodies, and to answer to the cell-wall of the resting Protococcus. This is termed the Zoogloea form of Bacterium.

Bacteria grow and multiply in Pasteur's solution (without sugar) with extreme rapidity, and, as they increase in number, they render the fluid milky and opaque. Their vital actions are arrested at the freezing point. They thrive best in a temperature of about 30° C. but, in most fluids, they are killed by a temperature of 60° C. (140° F.).

In all these respects Bacteria closely resemble Torula; and a further point of resemblance lies in the circumstance that many of them excite specific fermentative changes in substances contained in the fluid in which they live, just as yeast excites such changes in sugar.

All the forms of putrefaction which are undergone by animal and vegetable matters are fermentations set up by Bacteria of different kinds. Organic matters freely exposed to the air are, in themselves, nowise unstable bodies, and, if due precautions have been taken to exclude Bacteria, they do not putrefy, so that, as has been well remarked, "putrefaction is a concomitant not of death, but of life."

Bacteria, like Torula and Protococci, are not killed by drying up, and from their excessive minuteness they must be carried about still more easily than Torulæ are. In fact there is reason to believe that they are very widely diffused through the air, and that they exist in abundance in all ordinary water and on the surface of all vessels that are not chemically clean. They may be readily filtered off from the air, however, by causing it to pass through cotton wool.

I.

2.

LABORATORY WORK.

Infuse some hay in warm water for half an hourfilter, and set aside the filtrate: note the changes which go on in it—at first clear, in 24 or 36 hours it becomes turbid; later on, a scum forms on the surface and the infusion acquires a putrefactive odour.

Rub some gamboge down in water and examine a drop of the mixture with a high power: avoid all currents in the fluid and watch the Brownian movements; note that they are simply oscillatory-not translative.

3. Take a drop of fluid from a turbid hay infusionand examine it, using the highest power you have; in it will be found multitudes of

Moving Bacteria. Note their—

a. Form; elliptic or rodlike-sometimes forming short (2-8) jointed rows.

b.

Size; breadth, very small but pretty constant; length, varying, but several times greater than their breadth: measure.

C.

d.

Structure; an outer more transparent layer enveloping less transparent matter: in the compound forms the envelope appears only where two joints come in contact, so that the rod looks as if made up of alternating transparent and more opaque substances.

Movements; some vital, and some purely physical (Brownian). The former various but progressive: the latter, a rotatory movement round a stationary centre; study it in a drop of boiled infusion in which the Bacteria are all dead.

4. Treat with iodine-only the more opaque parts stain; probably then we have to do with protoplasm, enveloped in nonprotoplasmic matter.

5. Resting Bacteria. (Zoogloea-stage.)

a.

Examine the scum from the surface of a hay infusion; it exhibits myriads of motionless Bacteria, embedded in gelatinous material.

b. Treat with iodine; the Bacteria stain as before : the gelatinous uniting material remains unstained.

6. Mixed with the Bacteria proper, both in the pellicle and the fluid beneath, may be found the following forms of living beings.

a.

b.

Micrococcus. Bodies much like Bacteria, but short and rounded, and occurring singly, or in bead-like rows. They may be found free or in a Zoogloea stage.

Bacillus. Threads composed of straight cylindrical joints much longer than those of Bacteria