Turning once more to the present day, we will conclude with a brief glance round a physiological laboratory, and see by what methods the physiologist is preparing future surprises. The chemical department first claims our attention. The imports and exports of animals are carefully balanced, and the changes produced in the food examined. The animal is enclosed in an airtight chamber, air of known composition being pumped into it, and the air which escapes analyzed. The animal most used for this experiment is man himself, since he will take rest and exercise to order, the latter usually on the treadmill, by which it can be also measured, and can be relied upon not to while away the tedium of his imprisonment by gnawing holes in the walls or upsetting his food. All the substances used as food, found in or excreted by the body, are being thoroughly studied; but it should be remembered that this is chemistry, not physiology. Physiology is only concerned with protoplasm, and the physiologist who goes deeply into the chemistry of non-living matter has to discipline his mind against forgetting its ceaseless change, and trying to regard it as though it were constant. The actual chemistry of protoplasm will be a very hard nut to crack, and may defy us until we can depict molecules as well actually as we now can symbolically. Some idea of the difficulty may be formed if we consider that it is impossible to imagine a pure sample. From the restless activity which is the condition of its existence, it is always working changes in its surrounding, always mixed with raw material, and always masked by the products of its own metabolism. Even if we withhold the former, it consumes its own substance until the moment of death. It does not even look homogeneous under the microscope. Before, however, we can pursue the chemical methods further, it will be necessary to describe the histological. The reader may have already wondered how we managed to find out so much about the cellular structure of the body. It is no easy matter to cut up soft tissue, of the consistency of an unboiled egg, into thin slices which can be examined under the microscope. It is done in the following manner: The bloodvessels of the freshly killed body are injected with a fluid which instantaneously kills and fixes the cells in much the same way as an egg is fixed by being hard-boiled. The natural shape of the cells is thus preserved, and the loss of any of their chemical constituents by putrefaction prevented. The piece of organ is then impregnated with and cast in the middle of a solid block of paraffin wax, which is put into a machine and shaved up into thin slices, about 40,000 to the inch sometimes. One of these shavings is then stuck upon a glass slide, and on the wax being dissolved away with some such substance as benzine, a section of the tissue, about one cell thick, is left on the glass ready for microscopic survey.