The protective value of clothing mainly depends (1) inversely on the thermal conductivity of the material, (2) on its power of absorbing water, and (3) on the arrangement of the fibres of the material in the cloth. The conductivity of various materials is given in Tables LVI. and LXI.

In order to calculate the coefficient of protection of clothing one must know the thickness of the material. Table LXII. compiled by Rubner gives further information regarding the density of the material and of the amount of air enmeshed in the structure. This layer of imprisoned air, as we have already mentioned, has a greater protective value than mere thickness of material.

A man entirely clothed in a garment of a total surface (s) of 19 × 103 sq. cm., of a thickness (t) of 75 × 10−2 cm. and having a difference of temperature (d) on the two sides of 10° C., loses heat (Q) as shown by the following formula:

If the garment were of wool, c = 68 × 10-6, we have 68 × 19 × 36 × 24 × 10 -t × 103 × 102 × 10

The coefficient of utility of clothing materials may be determined by covering with them a number of similar copper spheres filled with warm water and finding how long they take to cool through 10° C. If the time taken to cool 10° C. by an unclothed sphere with an external temperature of 12° C. be taken as 0, and the time necessary for the same sphere (clothed) to cool to the same extent be t, then t/0 = U (Coefficient of utility).

The efficiency of clothing depends in great measure on the arrangement of the fibres of the cloth so as to enmesh air and thus form a stationary layer between the body surface and the outer air. The following table (Table LXIV.) from Lefèvre gives an idea of the protective value of clothing. His subjects were placed in a rectangular box through which a measured amount of air was passed. The temperature of the air just before it reached the body and immediately after leaving the body was taken. If M,

COEFFICIENCY OF UTILITY OF CLOTHING

353

the mass of air passing in t mins., is heated by 0°, then the heat

It will be noticed that heat production increases with the lowering of the temperature of the wind and also with increasing the speed of the wind. It is obvious that, under similar conditions, the heat lost (and the heat produced) by the clothed is less than that of the naked subject.

The colour of clothes has also something to do with their protective action. Quite apart from psychological effects, certain colours conduce to warmth and others to coolness. This is a problem that exercised the minds of those responsible for the health of white troops in tropical countries. The final result of one series of investigations was to recommend the wearing of white clothes with a black lining. The student may find it an interesting exercise to furnish a reason for this.

CHAPTER XXXII

TROPISMS THE SLAVES OF THE LAMP

"Behold the child, by Nature's kindly law,
Pleased with a rattle, tickled with a straw:
Some livelier plaything gives his youth delight,
A little louder but as empty quite ;

Scarfs, garters, gold, amuse his riper stage,
And beads and prayer-books are the toys of age,
Pleas'd with this bauble still, as that before,

Till tir'd he sleeps, and life's poor play is o'er."

POPE.

It may be laid down as axiomatic that once an object has come IT into equilibrium with its environment, it will remain so until some change in the environment disturbs the harmony. In other words, matter has inertia-moves when it is moved, stops when it is stopped, and alters only in so far as it is altered. All inorganic substances are not in equilibrium with their environment. Radioactive minerals, as we have seen, are characterised by undergoing considerable change seemingly independent of the nature of their surroundings. It is a moot point whether living things may be brought under this general statement. That they have inertia both in the ordinary physical sense and functionally is undoubted. Moreover, that alterations in the distribution of energy in the environment do lead to apparently corresponding alterations in the organism will be granted by most workers in this field, but all are not agreed as to how far the interpretation can be applied to animals high in the scale.

I. One of the best investigated phenomena in this line of study is that of heliotropism or phototaxis. It is well known that radiant energy is capable of influencing the rate of some chemical reactions-in proportion to the intensity of the light. This is known as the Bunsen-Roscoe Law, which may be formulated as:

it = constant,

where i is the intensity of the light and t the time of exposure.

(1) Certain animals and free-swimming plant-organisms move towards or away from the source of light. These are said to be positively or negatively heliotropic respectively. Caterpillars of Porthesia chrysorrhoea are of the former class. They move towards the light and may starve, with abundance of food just behind them.

(2) If positively and negatively heliotropic animals are placed in a trough covered half with red and half with blue glass, those that are positively heliotropic collect at the blue end and the others at the red end of the trough. Red glass is practically opaque, as every photographer knows, to the photo-chemical rays of light. The most efficient rays for heliotropic reactions are (a) the blue between 460 and 490μμ and (b) the yellow-green between 520 and 530μμ. Now, most blue glass permits not only the passage of the blue rays but of the yellow-green rays also (cf. Fig. 1).

(3) That the heliotropic animal is oriented in relation to the source of light is shown by a simple experiment due to Loeb. Direct sunlight is allowed to fall from the upper half of a window on to a table and diffused daylight from the lower half on to the same table on which is placed a test-tube in such a way that it lies at right angles to the window and is illuminated over one-half of its length (room half) by direct sunlight and over the remainder by diffused daylight. Positively heliotropic animals are introduced into the sunny end of the tube. They promptly and invariably move towards the window, i.e. out of the sunlight into the shade towards the source of light.

(4) To explain these facts (and others) Loeb has put forward an interesting theory. "Animals possess photosensitive elements on the surface of their bodies, in the eyes or occasionally also in the epithelial cells of their skin. These photosensitive elements are arranged symmetrically in the body and through nerves are connected with symmetrical groups of muscles. The light causes photochemical changes in the eyes (or photosensitive elements of the skin). The mass of photochemical reaction products SO formed "influences the central nervous system and through this the tension or energy production of the muscles. If the rate of photochemical reaction is equal in both eyes, this effect on the symmetrical muscles is equal and the muscles on both sides of the body work with equal energy; as a consequence the animal will not be deviated from the direction in which it is moving. This happens when the axis or plane of symmetry of the animal goes