Page:Popular Science Monthly Volume 21.djvu/375

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either physically mingled or perhaps weakly held together by the feeble chemical affinities which belong to all massive molecules. This constitutes a slime-like mass which, like all chemical compounds, exhibits certain characteristic reactions by which it is clearly distinguishable from certain substances, and known to be closely allied to certain others.

Protoplasm has the power of absorbing water in varying quantities, so that it is sometimes soft and nearly fluid, and again hard and leathery, though ordinarily of a medium consistence and density best described by the term "slime-like" already employed. It is then a glairy, tenacious, semi-fluid substance, transparent, and generally colorless; and if not quite the homogeneous, structureless matter which it was long supposed to be, there is at least an entire absence of differentiation of structure quite comparable to the observed absence of localization of function. When it acts it acts en masse or indifferently, sometimes in one portion, sometimes in another, of its substance, for the production of its simple movements and for the bringing about of its protean forms. Now all mouth, and anon all stomach, at times all feet, and again all lungs, it fulfills Dryden's famous description, "Everything by starts, and nothing long," save that it is ever and always protoplasm.

Like other albuminous substances, it is coagulable by heat, by alcohol, and by mineral acids, and is similarly stained by iodine and by nitric acid. Living protoplasm possesses also certain fundamental properties by which it may be distinguished from dead protoplasm. Prominent among these properties, grouped under the single term "vital," may be mentioned first, excitability, or, as it is more commonly called, "irritability" by which is meant the power of responding to a stimulus. An amoeba suddenly brought in contact with some foreign body responds to the stimulus so received by certain characteristic movements.

The movements of protoplasm, however, can not always be thus traced to some external exciting cause. Watching a specimen beneath the microscope, portions of the mass may be seen to creep, or rather to flow, slowly away in fine threads uniting with other threads from different parts of the same mass, thus forming an irregular net-work. Or perhaps it thrusts out temporary feet indifferently from any part of its surface by means of which it creeps slowly about, and it draws them in again, returning to the somewhat globular shape which appears to belong to its quiescent state. These movements are spontaneous; that is, they originate in the mass and result from the essential constitution of this kind of matter; therefore protoplasm is described as "automatic" or self-acting, and even as having a will of its own.

The internal changes which bring about these movements are believed to be identical with those which occur in muscle-tissue under stimulation, producing the change of form in muscle-fiber known as