476 Mr. W. Sutherland on the 



Before going further we must consider the specific heat of 

 water. In this property water again shows its very excep- 

 tional character ; not only is the specific heat much larger 

 than would be expected by analogy, but it is remarkably 

 nearly constant over a large range of temperature. Evidently 

 we have to do, not with a true physical specific heat, but with 

 a very complicated thermal phenomenon involving dissociation. 

 To unravel this we must make estimates of the specific heats 

 of trihydrol and dihydrol. According to Kopp's rule the 

 molecular specific heat of H 2 ought to be made up of 2*3 x 2 

 for the hydrogen, and 4 for the oxygen, that is 8'6; and there- 

 fore the specific heat ought to be 8 6/18 = "48, which is near 

 to the "504 for ice, but not to the I/O for water. According 

 to Kopp's rule the specific heat of trihydrol ought also to be "48 r 

 unless some provision has to be made for the binding of three 

 H 2 molecules into one, although all analogy indicates that 

 it would be slight. Thus ice regarded as a polymeric form of 

 H 2 uncomplicated by dissociation has a latent heat in accord- 

 ance with Kopp's rule, and by contrast the large specific heat 

 of water appears to be due to consumption of heat in causing 

 dissociation. On examining the available data I find that 

 substances on melting show an increase of specific heat 

 varying from 20 to 50 per cent., with an average of about 25 ; 

 and therefore we should expect pure liquid trihydrol at 0° 

 to have a specific heat about "625. Now we know from 

 Regnault's determinations that even up to 190° the specific 

 heat of water increases but little ; and we therefore conclude 

 that at 200°, when most of the trihydrol has been dissociated 

 and the dissociation heat is of small account, the specific heat 

 of the tolerably pure dihydrol is TO. As the usual rate of 

 variation of the specific heat of liquids is about "1 per cent, 

 per degree, we may infer that pure dihydrol at would have 

 a specific heat about "83. This result seems to involve a large 

 violation of Kopp's rule, according to which polymeric forms 

 of H 2 ought to have a specific heat near *48. But the great 

 difference in the densities of trihydrol and dihydrol at makes 

 it probable that there is a decided difference in their specific 

 heats at 0°. As the best estimates that we can make at present,, 

 let us write for the specific heats 



Cl = -6(l + -00lt), (23) 



c 2 = -8(1 + -0010 (24) 



In the heating of a gramme of water we have to supply 

 heat to raise the temperature of p 1 parts of dihydrol and of p 2 

 parts of trihydrol, and also heat to dissociate some trihydrol 



