234 PRINCIPLES OF GENERAL PHYSIOLOGY 



molecules present, which diminishes considerably between and 100 ; a minimum 

 would therefore be expected. 



There are, however, certain properties left unexplained by the hypothesis in 

 this simple form. Rontgen (1892), considering what might be the nature of the 

 polymer formed at low temperatures, was struck with the idea that it ought to 

 show itself when the whole of the water was transformed into the polymer. But 

 when water is cooled it turns into ice. How then do the properties of ice coincide 

 with the requirements of the case 1 Take the density ; ice is more bulky than 

 water at 0, so that if we assume that ice molecules exist in liquid water, we 

 can explain the existence of a point of maximum density at 4. Thus : the 

 change of volume when water is warmed from to 1 is the result of two 

 opposite effects dilatation of the simple molecules, according to rule, and con- 

 traction, due to change of ice into water. The latter process is preponderant 

 at the lower temperatures, -but nearly absent at the higher, and a point will exist 

 where the difference between the two is the least. It will probably occur to the 

 reader that water, according to this view, is a colloidal solution of ice. We shall 

 see presently that a third component has to be added, namely steam. 



Since the presence of the large molecules of ice increases viscosity, we see why 

 this property of water increases unusually rapidly when the temperature falls. 



The compressibility behaves similarly, on account of the effect of pressure in 

 causing depolymerisation. This would of itself result in a diminution of volume 

 and be added on to the compressibility of the pure hydrol. 



There still remain some questions unanswered. Although the compressibility of water is 

 greater than that of hydrol, it is unusually small. Again, we have not yet an explanation for 

 the high dielectric constant, nor why ice is lighter than water. 



There is an interesting fact in connection with water which throws some light on all of these 

 problems. Water of all known liquids (except fused metals) contains the largest number of 

 molecules per unit volume. Thus, in gram-molecules per cubic centimetre : 



Water 55 



Bromine - - - 20 



Sulphuric Acid 22 



Hydrofluoric Acid - 49 



Benzene - - - 1 1 '5 



Heptane - 7'1 



Ammonia 37 



This means that there is less space between the molecules of water than of other liquids. 

 The low compressibility is doubtless explained by this. The dielectric constant also increases 

 rapidly as the molecular condensation of a substance increases. Finally, it is to be supposed 

 that the molecular forces, which permit the molecules of hydrol to press unusually closely 

 together, disappear when the new group constituting ice is formed, so that the latter occupies 

 the greater volume corresponding to that which might be called the normal volume of water. 

 It is to be admitted, nevertheless, that the reason why water is such a closely packed liquid 

 has not been explained. 



As to the actual number of molecules existing in the various polymers, opinion 

 is still divided. The balance of evidence appears to be that ice is trihydrol, steam 

 is monohydrol, liquid water is mostly dihydrol with varying amounts of the other 

 two polymers according to the temperature. A curious fact is that, according to 

 Nernst and Levy (1909), there are still some polymerised molecules in water 

 vapour, so that, if these are identical with ice, it seems that we must admit the 

 presence of ice in steam ! 



There is also difference of opinion as to the relative number of molecules of ice present in 

 liquid water at various temperatures. As J. Duclaux (1912) points out, it might be possible 

 to attack the problem by the determination of the absorption of light of different wave lengths 

 by water and by ice. It appears that ice is much bluer in colour (than water, which is stated 

 to have, as dihydrol, a very pale green colour. The reader will probably have noticed that the 

 ice of glaciers is of a deeper olue than that of the same depth of water. 



It was incidentally mentioned above that it is necessary to introduce steam, as 

 a third component, into the water system, so that water in its ordinary liquid state 

 is a ternary mixture. This has been shown by Bousfield and Lowry (1910) by 

 comparison of the properties of water with a series of aqueous solutions of which 

 it may be regarded as the limit of dilution. The careful study of " solution 

 volumes " of caustic soda at different concentrations and temperatures showed that, 

 in addition to the abnormality of water near the freezing point, there is a second 

 in the neighbourhood of 60 and that the factor responsible for this effect becomes 

 more and more obvious as the boiling point is approached. The complete evidence 



