earth. The following table gives the mean temperature for the tenday periods ending on the dates given in column 1 and at a depth of 1 foot below the surface of the ground. Temperatures are given by them for other depths, as also for the air; the total rain and snow is also given. An investigation of the connection between earth temperature and the development of vegetation is being carried on by them, but as no results have as yet been published I give merely their soil temperatures at a depth of 1 foot, which usually agree, within a degree centigrade, with the average temperature of the air for ten days.

Mean temperature of the soil at a depth of 1 foot for periods of ten days at Montreal, Canada.

This series seems to show the powerful influence of a snow covering to keep the ground from cooling to very low temperatures during the winter. The minimum temperatures at 1 foot depth were -0.5° F. during the twenty days March 22 to April 10, 1889, and +0.4° F. during the ten days March 17 to 26, 1890.

METHODS OF MEASURING SOIL TEMPERATURES.

As it is very important that there should be numerous observations of soil temperature available for agricultural study, and as many persons are deterred by the expensiveness of the deep-earth thermometers, I would call attention to the fact that agriculture does not need to consider temperatures at depths below 4 feet and that the inexpensive, excellent system of thermometers, made by Green, of New York, has been recognized as the standard at stations in the United States; but for accuracy and convenience nothing can exceed the thermophone devised by Henry E. Warren and George C. Whipple, of the Massachusetts Institute of Technology.

Several methods of measuring deep-earth temperatures have been most thoroughly studied in the memoirs of Wild and Leyst, of St. Petersburg, a summary of which I have prepared and will submit at another time.

The soil thermometers constructed by Green are made in accordance with suggestions made by Milton Whitney, of the South Carolina Experiment Station, and have been used by him.

Whitney has published a description of this new self-registering soil thermometer as follows (see Agr. Sci., Vol. I, p. 253; Vol. III, p. 261):

This is a modification of Six's form of thermometer in which the maximum and minimum temperatures are registered in one and the same instrument. The essential features of the thermometers are as follows: A cylindrical bulb 6 inches long, filled with alcohol. The bulb is protected by a somewhat larger cylindrical metal tube, containing numerous holes, and is to be placed 3 inches below the surface of the soil-i. e., so that the bulb will extend vertically between the depths 3 and 9 inches, respectively, in the soil. The tube carrying the alcohol extends some 6 or 8 inches above the surface of the ground, when it bends twice at right angles and descends again to the surface, bends at right angles twice, crossing the main stem, and is carried up about 6 or 8 inches again, where it terminates in a bulb partially filled with alcohol. The lower bend in this stem carries a column of mercury which is drawn back toward the bulb when the alcohol contracts, and pushes a steel index up to the minimum temperature on a scale which reads downward. This index is held supported in the alcohol by a little spring when the alcohol expands and the mercury leaves it, while another index is pushed up to the maximum temperature by the other end of the column of mercury. The indices are set by the help of a magnet.

The advantages claimed for this instrument are that it gives at once, without any calculation, the mean temperature of a definite depth of soil, for which we now use at least three thermometers, while it gives in addition the maximum and minimum temperatures, and need only be read once a day instead of three times, as at present.

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Thermometers can be made, of course, with bulbs longer or shorter than the one described. We adopted the length of 6 inches placed 3 inches below the surface, as in our experience that represents a layer of soil in which most of the roots of the cotton plants are contained. We expect to distribute a number of these instruments through the State South Carolina] and have records kept for us near signalservice stations in our typical soils-a method which could hardly have been arranged with the old form. The instrument is mounted on a neat metal backing, and is made by H. J. Green, of New York. It cost $10 without packing or express charges. The great trouble about the instrument is the danger in transportation of having the index get down in the mercury column. For this reason it has to be transported in a box on gimbals to swing freely within a larger box, so that it will always remain upright. We had such a box made, capable of carrying eight or ten instruments, for $5.

From experiments at Houghton Farm (Agr. Sci., Vol. II, p. 50) F. E. Emory finds that the thermoelectric couple and galvanometer, as used by Becquerel, consumed much time and was frequently useless owing to atmospheric electricity and ground currents. Shortstem graduated thermometers, with bulbs immersed in oil and fastened at the lower end of a light wooden rod, gave good results when the temperature at the thermometer was not warmer than that of the overlying soil or the atmosphere; otherwise a circulation of air takes place. He finds that the telethermometer, giving a continuous record, answers his needs, but we know nothing of its accuracy.

T. C. Mendenhall (1885) describes a modified form of thermometer for observing the temperature of the soil at any depth, which he calls the "differential resistance thermometer." Experiments with this instrument at Washington, D. C., have shown him that it is much less troublesome than Becquerel's electric method, but still too troublesome to be recommended to any but persons accustomed to electric measurements. Mendenhall's arrangement consists essentially in utilizing the varying resistance of a platinum wire which extends from the upper end of an ordinary mercurial thermometer down into its bulb. The total resistance diminishes as the temperature rises and allows the current to flow through less platinum but more mercury. The changes in the resistance are measured by the galvanometer, but he hopes to substitute for this the telephone, which will make the apparatus more convenient for general use.

[It is desirable that this or Becquerel's method or the thermophone be provided in connection with the ordinary buried longstem thermometers in order that by an annual or more frequent set of comparative observations the changes in the zero point of ordinary thermometers may be detected.-C. A.]

Chapter IV.

THE INFLUENCE OF SUNSHINE ON ASSIMILATION AND TRANS

PIRATION.

CHEMISTRY OF ASSIMILATION (ABBOTT).

The atmosphere is composed of about 79 per cent of nitrogen and 21 per cent of oxygen when we consider their volumes, but 77 per cent of nitrogen and 23 per cent of oxygen when we consider their relative weights. With these gases there are mixed small quantities of carbonic-acid gas, ammonia, hydrocarbons, and other impurities. With this" dry atmosphere" there is intermixed a very variable quantity of aqueous vapor or moisture, which in extreme cases may amount to as much as 5 per cent, by weight, of the dry air. These are the elements that are to be compounded by sunshine and heat in the laboratory of vegetation.

By respiration the leaves of plants, when in the dark, absorb oxygen from the air and set free carbonic-acid gas.

By assimilation, as shown by Garreau, these same leaves in the sunshine absorb carbonic-acid gas from the air and set free oxygen, retaining the carbon in new compounds. Assimilation is a process of greater intensity than respiration. Respiration is a process analogous in its results to that occurring within every animal organism, but assimilation is a process peculiar to the plant life.

By transpiration the leaves rid themselves of the superfluous water that, as sap, has served its purpose in the process of assimilation by bringing nourishment from the soil and delivering it up to the cells of the plant; a small portion of the nourishment and of the water may have been absorbed by the cells in the trunk of the tree, the stem of the vine, or the stalk of the grain and grass, but the majority of the water is removed by transpiration at the surface of the leaves in order to make room for fresh supplies of sap. Some water always remains in the cells of the seeds and grains until they are dried after maturity, but a well-dried crop contains relatively little water. This transpiration is stimulated by, and almost entirely depends upon, the action of sunshine on the leaves; it precedes evaporation.

Evaporation is not transpiration; the former takes place from the surface of water existing either in the moist earth or in films on leaf surface or in larger masses, while transpiration takes place through the cell wall and is a process of dialysis, an endosmosis and exosmosis

by which the cell takes in the sap, retains what it needs, and then gets rid of the water and the dissolved substances which it does not need. Thus the cell wall thickens and enlarges and the contents of the cell increase. The sap enters the cell from that side of the cell which is turned toward the interior of the plant or adjacent cells, and the rejected water penetrates the cell wall on that side of the cell which is exposed to the open air, and especially on that side exposed to the sunshine; having reached the outer surface of the cell wall on this side of the cell it is then evaporated. This endosmosis by which the sap enters the cell on one side, and the exosmosis by which it leaves the cell on the opposite side, constitute the fundamental mechanics of all vital activities; the chemistry of animal and vegetable life differs from the ordinary chemistry of the laboratory in that the former studies the behavior of the cell wall toward the molecule, while the latter studies the behavior of the molecule toward the molecule. An interesting contribution to the development of this idea of the chemistry of the action of the cell is contained in two papers by Miss Abbott (now Mrs. Michael, of Philadelphia), published in 1887 in the Journal of the Franklin Institute; from the second paper I take the following extract:

The botanical classifications based upon morphology are so frequently unsatisfactory that efforts in some directions have been made to introduce other methods.

There has been comparatively little study of the chemical principles of plants from a purely botanical view. It promises to become a new field of research.

The Leguminosæ are conspicuous as furnishing us with important dyes, e. g., indigo, logwood, catechin. The former is obtained principally from different species of the genus Indigofera, and logwood from the Hæmatoxylon campechianum, but catechin from the Acacia catechu.

The discovery of hæmatoxylon in the Saraca indica illustrates very well how this plant, in its chemical as well as botanical character, is related to the Hamatoxylon campechianum; also, I found a substance like catechin in the Saraca. This compound is found in the Acacias, to which class Saraca is related by its chemical position as well as botanically. Saponin is found in both of these plants, as well as in many other plants of the Leguminosa. The Leguminosa come under the middle plane of multiplicity of floral elements, and the presence of saponin in these plants was to be expected.

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From many of the facts above stated, it may be inferred that the chemical compounds of plants do not occur at random. Each stage of growth and development has its own particular chemistry.