Page:Popular Science Monthly Volume 32.djvu/87

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it lives in water and has a body so constituted as to readily permit the inflow and outflow of water and solutions of salts, may be easily used for experiments. The results of the experiments have a practical as well as scientific interest, since they confirm the common explanation of the increase in bulk of oysters by "floating," and show that it is essentially a process of watering in which the bulk is increased without any corresponding increase, but rather, if anything, a loss of nutritive material.[1]

It is a common practice of oyster-dealers, instead of selling the oysters in the condition in which they are taken from the beds in salt water, to first place them for a time—forty-eight hours, more or less—in fresh or brackish water, in order, as the oyster-men say, to "fatten" them, the operation being called "floating" or "laying out." By this process the body of the oyster acquires such a plumpness and rotundity, and its bulk and weight are so increased, as to materially increase its selling value.

The belief is common among oyster-men that this "fattening" is due to an actual gain of flesh and fat, and that the nutritive value of the oyster is increased by the process. A moment's consideration of the chemistry and physiology of the subject will make it clear, not only that such an increase of tissue-substance in so short a time and with such scanty food-supply is out of the question, but that the increase of volume and weight of the bodies of the oysters is just what would be expected from the osmose which would naturally take place between the contents of the bodies of the oysters as taken from salt water and the fresh or brackish water in which they are floated.

If we fill a bladder with salt water, and then put it into fresh water, the salt water will gradually work its way out through the pores of the bladder, and, at the same time, the fresher water will enter the bladder; and, further, the fresh water will go in much more rapidly than the salt water goes out. The result will be that the amount of water in the bladder will be increased. The bladder will swell by taking up more water than it loses, while at the same time it loses a portion of the salt.

It does this in obedience to a physical law, to which the terms osmose and dialysis are applied. In accordance with this law, if a membranous sac holding salts in solution is immersed in a more dilute solution, or in pure water, the more concentrated solution will pass out, and at the same time the water, or more dilute solution, will pass in, and more rapidly. The escape of the concentrated and entrance of the dilute solution will be, in general, the more rapid the greater the difference in concentration and the higher the temperature of the two solutions. After the osmose has proceeded for a time, the two solutions will become equally diluted. When this equilibrium between

  1. The following statements are adapted from a paper presented to the last meeting of the American Fisheries Association.