The following table, also from•Becquerel, gives the relative retention of heat by different soils, that of calcareous sand being one hundred, and also the time of cooling from 144°. 5 to 70°.2, the temperature of the surrounding air being 61 °.2, of eighteen feet pubes of the same size of the dif• ferent earths: Another noteworthy agricultural fact in relation to heat is, the amount of heat which falls upon a given surface depends upon the inclination to the different points of the horizon. A field, for instance,. in latitude 40°, sloping toward the south, receives a greater, and one toward the north a less amount of heat; moreover, the former obtains more than an equal extent of pound parallel to the horizon, and the latter, as in the other case, much less. A field, also, which slopes in an easterly direction receives less heat than another inclined toward the west, inasmuch as more reaches the latter, since the maximum heat of the day takes place after the sun has passed the meridian; as it is, each of these enclosures gets a less amount than one of equal extent parallel to the horizon. A certain degree of heat is required to bring the sap of plants into active play, just as a certain amount of heat is required to produce germination in seeds, and each species of plants has its own determinate degree of mobility of sap or germi nation, as respects the seed. (See Germination.) Thus vegetation is accelerated and becomes luxuriant in pretence of its maxima of heat, act ing upon rich earth in connection with moisture. Hence, we see gardeners forcing plants out of season by means of heating material, in connec tion with glass structures, or by means of hot beds and cold frames, and even • by the protection of hedges, fences, and belts of timber. In rela tion to the heat of the earth, Pouillet, from many observations, gives the following deductions: ,The diurnal variations are not perceptible at depths greater than about forty inches. s The mean annual temperature of the different strata differs little from the mean annual temperature of the air. The differences between the maxima and minima of the different strata decrease in a geometrical progression, while the depths increase in an arithmetical progression. From all the observations it appears that, at a depth of from twenty-six to twenty-nine feet, the annual varia tion is only 1'.8 Fahr.; at from forty-nine to fifty-two feet, it, is but 0°.18 Fehr.'; and at a depth of from sixty-five to eighty-one feet, it becomes only 0°.2 Fahr. tAt the depth of about twenty-six feet, or where the variation is 2° Fahr., the seasons are precisely reversed; that is, the maximum temperature occurs about the 1st of January, and the minimum about the end of June. In animal tissues heat is generated by the food we eat and the exercise we take. In vege tables chemical action produces but little heat, the heat of vegetables being principally due to the heat surrounding them and their varying powers of conducting heat. The following observations, collected from various sources, will show the researches that have been made from time to time in thislirection: In the tissues of living bodies a multitude of chemical processes are constantly going on, engendering a certain heat, which in warm-blooded animals constitutes their temperature proper. But in cold-blooded animals and in vegetables, this chemical produc tion of heat being very feeble, the temperature which they have comes principally from the ambient medium; not only from the air but also from the water, charged with different elements, which is sucked up by the roots in order to form the sap. Like that of the air, the temperature proper for vegetables is submitted to periodical variations, only its amplitude is much less extended. This heat is liberated in the leaves, organs of respiration, in the flowers, organs of generation, as well as in the green parts of vegetables. In the phenomena of nutrition, the ascent of sap can not be effected without a slight production of dynamic heat, nor afterward can its elaboration in the leaves, under the influence of light, take place without chemical reactions, and consequently without another liberation of heat. In fine, the whole of these chemical reactions are always accompanied by the pro duction of heat, whose influence upon the vital functions of the plant we have not yet been able to appreciate. This estimation becomes the more difficult, as the production of vegetable heat is accompanied by many effects of cooling, such as evaporation from the leaves, liberation of oxygen, arising from chemical reactions which take place under the influence of light, and from which results a disengagement of carbonic acid.. The first experiments undertaken to determine the temperature of the internal parts of vegetables do not go back much beyond eighty years. At that time thermometric observations did not possess that degree of exactitude which they have since acquired. None of the precautions necessary to protect the thermometers from atmospheric influences were then in use. Hence resulted a multitude of errors concerning the true nature of the internal heat of vegetables. In the first place Hunter's experiments, made in 1775, upon a walnut tree, by means of a hole which he had bored in it, in an obliquely downward direction toward the center. The sap of the walnut, which flowed abundantly, was frozen at 0° when it was drawn from the tree, although. the tem perature of the latter was lower. Why does the sap, he asked, preserve its fluidity in its natural channels much below zero? He cites the prop erty possessed by water of freezing more easily when in a considerable raass, than when it is found in capillary spaces, where the attraction exerted by the sides upon the liquid molecules opposes their solidification. Indeed, Senebier has seen water remain liquid at 7° below zero in capillary tubes, with a diameter greater than that of the ducts of plants. Neuffer and Schil bler have found that trees freeze with more difficulty, as their layers are closer. This circum stance contributes to give vegetables the power of resisting a high degree of cold. Rumford and Leslie have also made the observation that air being a bad conductor of heat when its molecules can not be displaced, must be the best envelope we can take to oppose refrigeration. It seems to result from this that the more layers possessed by vegetables the better ought they to resist cold. Such is the case with plants having a large number of epiderms, as the birch, which attains the highest elevation in the Alps, and advances farthest into the polar regions, and as the horse chestnut tree, which flourishes in the tropical regions. Numerous observations have been made by Pictet and Maurice at Geneva, from 1796 to 1800, upon a horse-chestnut tree, in the trunk of which a hole had been bored on the north face to receive a thermometer. The results were as follows: During the five years from 1796 to 1800 the annual mean temperature of the air on the north side was sensibly equal to that of the tree. The annual difference between the two means being only 0.04 of a degree, we may infer that the mean temperature of the tree was strictly equal to that of the air. The difference between the monthly temperatures of the air and those of the tree was more or less sensible according to the season. During the winter months the temperature of the earth at a little over four feet below the surface of the ground is higher than that of the tree; in spring, it is the reverse. In summer, according to the year, the temperature of the earth is higher or lower than that of the tree; and in autumn it is always higher In general, it is toward autumn the temperatures are nearly equalized, but varying with seasons. Although the mean temperature may be the same in the air and in the tree, nevertheless, the varia tions that are met in the tree are much less than those in the air; the tree, therefore, preserves longer its acquired heat within limits depending upon its diameter. Another important fact shown by the Geneva observations is that the imum of temperature in the air, according to the season, takes place from two to three o'clock p.m., while in the tree the temperature continues to rise, however feebly, from two o'clock until sunset, when the observations were ended. Thus the maximum in the tree appears later than in the air. Moreover we need only take the mean of the tree's temperature at sunrise and sunset in order to obtain almost exactly the mean tem perature at two o'clock. During the three years, 1796, 1797, 1798, the variation of temperature through the night, in the air, was at a mean of 3.42, and in the tree of 0.73; that is to say, 4.69 times greater in the first case than in the second. In 1826, Haider advanced the assertion that trees are sometimes found in winter at a temperature below the freezing point, and pass even to a con gealed state without injury to their vitality. The thermometer had descended almost to-15°, and —17°.07 in some young trees, without hindering their vegetation. Haider attributes to evapora tion the lower degree of temperature observed in the tree. Rameaux has reached the following conclusions :—The temperature at any altitude at the center of the trunk of a poplar tree increases during the day and diminishes during the night; always differing in one section from another, according to the thickness. Before sunrise, and even for some time afterward, the central tem perature decreases from the foot of a tree' to its top. The contrary takes place during the rest of the day.—In the daytime, the temperature of a section is carried as far beyond the temperature of inferior sections as the ambient heat is stronger. These differences reach their maximum about sunset, after which they diminish gradually, are effaced little by little, and end by taking contrary signs.—During the night the temperature of any section was so much higher than that of the sec tions situated over it as this was lower than the ambient temperature. The differences reached their maximum toward sunrise, after which they diminished very rapidly, ending by taking contrary signs. In the morning, before sunrise, the central temperature of the tree in its four sections was inferior to that of the ground at the mean depth of its roots; during the day it was the contrary. Rameaux finally reached the following conclusion: Atmospheric heat, if not the sole source, is so predominant a cause of vegetable temperature that its effects weigh down the other causes put together. Whence it follows that the temperature of a tree must, in each section, increase from surface to center during the daytime, when the ambient heat is highest; it must diminish, on the contrary, from centre to surface during the night. The author, nevertheless, believes that the temperature of the ascending sap must exert some influence equally with that of the air. In the polar regions we observe the same facts. The experiments made at Bossekop, and by Thomas, at ord, in the winter of 1839-40, have proved that heat follows in the interior of pine trunks the curve of the temperatures of the air, with a retardation of eight to twelve hours When the diameter of the tree was increased the retarda tion was greater. We can conclude from all these experiments, adds Becquerel, that cold penetrates to the heart of living trees, as to that of dead ones;. that in the living pine the tem perature throughout the greatest cold is a little more elevated, either because the sap sometimes. rises even through mean temperatures of the air Inferior to zero, and heats the interior of the tree by becoming congealed, or because the heat of the ground being superior to that of the air, gives the sap a portion of its proper heat. Ac
cording to Becquerel, the observations hitherto collected upon the temperatures of vegetables, lead to the following consequences: The annual mean temperature of vegetables is. the same as that of the air; the two curves of temperature have the same appearance, although not coincident throughout, considering that trees only participate in the diurnal variations of the temperature of the air in proportion to their dia meters. The air is, therefore, the principal source of vegetable heat. The maximum of temperature in the air is reached in winter about two o'clock in the afternoon, and'in summer about three o'clock; in vegetables these hours are delayed according to their bulk. In trees with a diameter of three or four dkimbtres (twelve to fifteen inches) the maximum occurs in winter about nine o'clock p.m., and in summer about midnight. When the temperature of the air sinks below zero, vege tables resist for a longer or shorter time this. cooling, as well as the heating which follows a thaw, without being due to the bad conducting properties of the wood. When the cold con tinues during many months, as in the north of Europe, the temperature in the tree is succes sively lowered, but never as far as in the air. There is a difference of from one-half to a degree. The temperature of vegetables, which is almost wholly derived from without, appears, neverthe less, to be influenced by the heat liberated in chemical reactions in the tisErue§. and by the temperature of the soil from which their roots extract liquids to form the sap. We are still ignorant how, in winter, when the upward motion of the sap is almost suspended, the ground temperature may diminish refrigeration, although the external temperature is below zero. Trees exposed' to solar and to nocturnal radia tion, during the day and a great part of the night, beat the strata of air with which they are in contact, and cool them when the leaves have taken the temperature of the air before the sun has yet appeared. But its cooling effect is small under the latitude of Paris, at least during the spring:, at which time the observations were made. Woods and forests probably exert the same influence, varied by divers causes; but they really act as repositories of heat when the trees. receive the direct rays of the sun, which ie prin cipally from May to October. The air, as a transparent body, only stops a small part of the solar rays in their passage through it. These rays then proceed to opaque bodies, the,earth, and to plants, which absorb a much more con siderable part of them. Solar heat is, therefore, one of the principal elements which distinguish agricultural climates as they receive it more or less abundantly, either by reason of latitude or altitude of the localities, protection from winds, the exposure, or the topographical character of the soil, and of a multitude of atmospheric agents which reflect or intercept the solar rays. As an effect purelylocal, science at first neglected it; but the savants were not slow in perceiving the enormous influence of this solar heat upon the progress of vegetation, maturity of fruits, etc. Humboldt did not cease to remind us that.
solar effects must be studied in order to account for vegetable phenomena, and the Academy of Sciences of Paris has made it a subject of its recommendations to travelers. In 1840 Gasparin made some experiments upon three mulberry trees of the same variety, the first receiving full solar rays, the second receiving them only till noon, and the third kept entirely in the shade. The solid matter of the leaves of the first had a weight equal to 0.45 of the whole leaf, that of the second 0.36, and that of the third 0.27. In 1852 he cultivated beans on a plat of ground divided by a partition which shaded one-half of it from the solar rays. After drying, the plants from the south weighed 0.581; the same number of plants grown at the north, although much more developed in height, 0.337; but it was in their fructification that the difference was remarkable; the southern plants had one hun dred and thirty-one pods, the northern only forty-seven. It is impossible, adds Gasparin, to attribute these results to a simple augmentation of heat; light enters into it for the most part, combining its influence with that of caloric. Indeed, in this experiment the plants had re ceived in eighty-four days a sum of 1,286°.50 atmospheric heat and only 255°. 71 of solar radiation. Certainly an addition of 3°.07 of obscure heat received each day in a greenhouse could not have produced such results. The incontestable effects of radiation upon vegetation struck De Saussure, the great observer of the Alps, and he sought to estimate it by means of experiments upon heat condensed at the bottom of a glass box. Pouillet has estimated the quantity of solar heat which reaches the limit of the atmosphere at 1°.7633 per minute. By supposing the sun at the zenith, he has found for Paris the mean 0.7390 as the number of rays which reach the earth. Forbes has found in Switzerland 0.680, and Quetelet, at Brussels, 0.615. But what is especially important to know, for agricultural practice, is the quantity of solar heat which strikes opaque bodies and accumulates there, by the great principle of conservation of forces, together with the variable state of the temperature of these bodies exposed to the sun, on different days of the year and different hours of the day. When we see solar radiation produce effects so diverse in opaque bodies according to their nature, volume, figure, color, etc., we see how difficult it is to know what happens in each structure of the vegetable kingdom. Thus wheat struck by the sun acquires a temperature different from that of grapes or melons; the leaves, a temperature other than the stem, flowers and fruits; the parts dry or deprived of life become warm very differently from living bodies, whose surface con stantly transpires, and whose heat never much surpasses that of the surrounding air. Another mathematical and astronomical question which must be taken into consideration, is, that the sun strikes the earth's surface more or less obliquely, and acts upon it according to the sines of the angle of incidence, and, while ovoid bodies are fully affected by these changes of inclinations, spherical bodies are not at all. We, therefore, perceive what importance this capital question has in regard to the amount of solar radiation received by the soil, by leaves or fruits, according to their extent and form, relatively to the law of the sines of the angle of incidence.
A late report of the chemist of the Department of Agriculture, Washington, gives some facts in rela. tion to loss of heat by radiation and the influence of water by evaporation, from which we gain the following: If we examine the tables showing the difference of the temperature 'of the air and the dew-point, we find that it varies widely with the proportion of moisture in the atmosphere. Thus, when the relative humidity is seventy and the temperature of the air is 70°, the difference between the latter and the temperature necessary to the formation of dew has been found to be 10°. When the relative humidity is fifty and the temperature of the air 73°, the variation 20°; while if at the same air temperature the relative humidity be forty-three, the variation will be 25°. With the relative proportions of moisture here given, and with higher tempera tures, these variations become still wider. We see, therefore, that during the dry weather that often occurs during •the summer months, vege tation suffers not only from deficient moisture to supply its demand, but also from very much wider variations of temperature due to radia tion. Indeed, it is during the prolonged summer drbughts that we generally notice the heaviest dews. Now, let us see what will be the temperature to which the plant must be subjected in conse quence of this reduction due to radiation. Wells found that a thermometer laid upon a grass-plot on a clear night sank sometimes 14° lower than a similar thermometer suspended in free air at a height of four feet above the grass; and the obser vations of Pouillet seem to show that the diminu tion of temperature attending the production of dew is at all times sensibly constant at about the figure determined by Wells. Our summer-night temperature usually varies between 70° and 85° Fahr. With the reduction which must take place when the relative, humidity is seventy, we will have temperatures ranging from 60° to 75° ; with the relative humidity of fifty we will have tem peratures of 50° to 65°, while with the relative humidity of forty-three, which could, of course, occur only during very dry weather, the tempera ture would fall from 45° to 60° Fahr. But these are considerably below the lower limits of tempera ture at which it has been found by careful obser vation the movements of protoplasha and the transformation of plastic material can take place, and which, for even half hardy plants as the Phaseolus (bean) and Zea mais (corn), is stated at about 61° Fahr. The lower limit for the more tender plants is doubtless much higher than this, so that it is easy to see how they may suffer and become debilitated by the cold of summer nighsa. But the germination of fungus spores and the growth of mycelium, may take place at a much lower temperature and during the temporary interruption of the transformation of plastic material, which must occur at the reduced tem peratures mentioned. The mycelium of the ger minating spore may effect penetration through the epidermis of the plant into the cellular tissue beneath, where it may feed upon the dormant protoplasm, which is already fitted for assimila tion. This result having been arrived at, the recovery of the plant from the abnormal condi tion produced by the reduced temperature, which would otherwise take place, will be prevented, leaving the plant in condition favorable to the rapid growth and spread of the fungus even to the death of the plant. But while the discoveries of Wells and his colleagues showed under what condition radiation of heat from the plant into space and the production of dew could take place, they also showed how it could be prevented, and in this they give us a remedy for one of the pos sible causes of mildew and rot. They found, what is familar to all, that dew is not formed on cloudy nights nor in sheltered positions. Indeed, one of Wells' experiments to show that the radia tion of heat took place, and that the formation of dew vent hand-in-hand with it, was to arrange a board or piece of pasteboard on props and note the temperature and the production of dew in the grass below it and on some cotton wool on the top of it. He found that beneath the cover no dew was produced, and there was at the same time no reduction of temperature. In relation to the second cause of reduction of temperature, evap oration, we have seen how debilitation of the plant, due to an interruption of the vital pro cesses by cold, may be produced. Quoting again from Sachs, we find that in the atrial parts of plants transpiration is an energetic addi tional cause of loss of temperature, inasmuch as water in the act of evaporation withdraws from the plant the amount of heat necessary to its vap orization, and hence makes it colder. If water upon a leaf be allowed to evaporate slowly, no injury results; but, if a current of dry air of ave rage temperature, or even warm air, be caused to pass over it, a reduction of temperature will take place often greater than will occur in the produc tion of dew.