504 AGRICULTURAL REPORT.

It is shown by repeated observations that alternations of freezing and thawing are more hurtful to the tender plant than a uniform con-

tinuation of cold; whether this is produced by an action analogous to-

that we have described in reference to the water-pipe, or is due in part to this and other changes, we are unable to say. When, how- ever, the sap of a plant, killed by frost, is examined with a micro- scope, we find in it portions of destroyed tissue. It has also been observed that air may sink a few degrees below the freezing point without injury to the plant, provided the air at the time be very dry. It would seem from this that the freezing of the vapor, and the pro- duction of the minute crystals which constitute hoar frost, are, in a degree, essential to the effect.

As a general deduction from chemical and mechanical principles, we think no change of temperature is ever produced where the actions belonging to one or both of these principles are not present. Hence, in mid-winter, when all vegetable functions are dormant, we do not believe that any heat is developed by a tree, or that its inte- rior differs in temperature from its exterior further than it is pro- tected from the external air. The experiments which have been made on this point, we think, have been directed by a false analogy. During the active circulation of the sap and the production of new tissue, variations of temperature belonging exclusively to the plant may be observed; but it is inconsistent with general principles that heat should be generated where no change is taking place.

All animals, so long as life continues, generate heat, and have tem- peratures peculiar to themselves. In the higher class of air-breathing animals this temperature varies within comparatively slight limits under the influence of motion, rest, or of external circumstances; and a reduction of temperature by the application of external cold, pro- duces, as it is well known, a sluggish condition, which finally termi- nates in death. The effect of external cold can be prevented by arti- ficial covering, or it may be obviated, in the case of domestic animals, by an extra allowance of food. The sagacious farmer is aware of the fact that a well-sheltered enclosure for cattle is not only a humane, but an economical provision.

Many observations have been made on the temperature peculiar to different animals, and a considerable number of observations recorded of a less scientific character in regard to the effect of the variations of temperature to which they may be subjected without permanent injury. The most astonishing fact, and one which could scarcely be believed, if we were not in this country familiar with it, is, that many cold-blooded animals can be actually frozen, and be to all appearance dead, and yet be revivified by gradually thawing in water near the freezing point.

Fish, as we are assured on reliable authority, are often brought to our Northern markets from a great distance in a frozen condition, and

may be restored to life by the process we have mentioned. This is a subject, as it appears to me, of high interest in a physio- logical point of view, and would richly repay the application of well-devised systems of investigation. Can it be possible that the

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