228 PRINCIPLES OF GENERAL PHYSIOLOGY 



EXPANSION BY HEAT 



The fact that water has its maximum density at a temperature of 4 above its 

 freezing point is familiar to all. Unlike most common substances, when cooled 

 from 4" to 0, instead of contracting, it expands. At the moment of solidification, 

 there is a further expansion, but this is not uncommon. The two phenomena 

 together account for the fact that large bodies of fresh water, when cooled, freeze 

 only on the surface. Since water at 4 is denser than at a lower temperature, 

 it will sink and no ice will be formed in the depths until it has reached them by 

 growth from the top. In salt water, of course, the ice that separates is free from 

 salts and is therefore still lighter than the sea water. 



If ice were formed in the winter at the bottom of lakes and streams, it would 

 never get melted in summer, since the process of diffusion of the warmer and 

 lighter water from the surface is so slow. An old experiment of Rumford's shows 

 that a test-tube of water frozen at the bottom can be boiled at the top without 

 melting the ice. In the lakes, the ice would become thicker every year, until 

 ultimately the whole, or nearly the whole, of the water would be turned to ice. 



So far for the thermal properties of water. The only other liquid which approximates to 

 it in the merely thermal properties, necessary for life as we know it, is ammonia, and even this 

 lacks the anomalous expansion before freezing. 



L. J. Henderson (1913) makes use of these characteristics of water, and there are other 

 exceptional ones, as we shall see, in order to illustrate his point of view that we must consider, 

 not only the adaptation of the organism to the environment, but also the fitness of the environ- 

 ment to the organism. Of course, in one sense, the adaptation of the organism to a particular 

 condition implies also that this condition is fitted for the organism, but there is an obvious 

 distinction to be made, since the organism is capable of change in response to changes in the 

 environment, while the converse does not occur. None the less, it is a remarkable fact that 

 the properties of the substances everywhere present, such as water and carbon dioxide as also 

 those of carbon itself, are just such as to allow the most varied and complex chemical and 

 physical systems with which we are acquainted, and call by the name "vital," to be evolved. 

 No doubt, the mix of the question lies in the words "call by the name vital." In a world in 

 which liquid ammonia took the place of water, another kind of complex organisation might 

 have been developed ; although, it must be admitted, it seems impossible that the complexities 

 and endowments of the "organisms" formed could ever reach the perfection of those whk-h 

 we know under the present conditions (see also the remarks on adaptation on page 201 above). 



SURFACE TENSION 



We pass on to consider some other of the physical properties of water. As we 

 have seen, its surface tension, 75 dynes, is higher than that of any other liquid 

 except mercury, although glycerol, 65 dynes, is not far below it. 



We have also seen, in Chapter III., the importance of this in relation to the 

 phenomena of adsorption, which play so large a part in physiological processes, 

 owing to the heterogeneous nature of the systems concerned. 



The supply of water from the soil to plants is greatly influenced by the large 

 surface tension of water, since it is thus enabled to reach the roots from a 

 considerable distance. It is said that, under ordinary circumstances, water may 

 rise in the soil as much as 4 or 5 feet. See the monograph by Russell 

 (1912, pp. 102-105). 



TRANSPARENCY TO RADIATION 



Water in the liquid state is practically transparent to all the rays of the 

 visible spectrum. In very deep layers it appears blue, which means that it 

 absorbs more of the rays of longer wave length than of the shorter. The rays 

 of still longer wave length, heat rays, are comparatively more absorbed, so 

 that a vessel of water is a fairly efficient method of absorbing the heat from 

 an arc lamp, used for purposes of microscopic observation or photography. Ultra- 

 violet rays are absorbed to a very small extent. 



This relatively small absorption of the energy of radiation is probably of 

 some importance in allowing the access of this form of energy to substances 

 in solution in water. Especially in the case of the green leaf, the light energy 

 must not be degraded to heat before reaching the photo-chemical system of the 

 chloroplast. 



