Molecular Constitution of Water. 479 



dihvdrol will require 177 + 189 or 366 calories, and for the 

 solid trihydrol 382 ; the usual so-called latent heat of evapo- 

 ration of ice is about 660 ; and thus the latent heat of eva- 

 poration of liquid trihydrol, if evaporated without dissociation, 

 would be 278 calories; of solid trihydrol 294. 



The latent heat of evaporation of trihydrol at its boiling- 

 point can be estimated in exactly the same way as that of 

 dihydrol. As a 2 (54:/p 2 )% at 0° is 1141, and diminishes 2*121 

 per degree till it is zero at the critical temperature, the 

 •critical temperature of trihydrol is 538° 0.; therefore at § 

 of the absolute critical temperature a 2 (54//3 2 )f is 271 x 2*121, 

 and 



I = 1209 x 271 x 2-121 x v 2 /54, 

 w 2 =l/-88(l--0002x267); 

 .'. I =15,440, with 10 6 dynes as unit. 



Taking the absolute boiling-point at § of the absolute 

 critical temperature, as it nearly is for most liquids, we find 

 for X by (29) at 267° G. the value 209. This is at 267°, 

 while at 0° on our former reasoning the value found was 278, 

 and these two are in reasonable agreement, implying about 

 250 at 100° C. 



The main results of this section can be expressed in the two 

 thermochemical equations 



2H 2 = (H 2 0) 2 + 189x2xl8 cal., 

 3(H 2 0) 2 =2(H 2 0) 3 + 177x6x18 „ 



6. TlieViscosity of Water. 



In its viscosity water shows one of its most characteristic 

 peculiarities, namely, the decrease of its viscosity with in- 

 crease of pressure, discovered by Rontgen, and discussed by 

 him in his theory of the constitution of water. We can 

 make this phenomenon the subject of quantitative study, 

 thanks to the refined measurements made by R. Cohen (Wied. 

 Ann. xlv.). But it will be best to take the variation with 

 temperature first. At 0° trihydrol is below its true physical 

 melting-point, and accordingly we should expect the viscosity 

 of the liquid trihydrol to be very great ; on the other hand, 

 dihydrol at is probably far above its melting-point, and 

 must have a relatively small viscosity ; therefore the viscosity 

 of water at temperatures near 0° must be mainly that of the 

 trihydrol in it, the dihydrol gradually becoming of more 

 importance as the temperature rises. 



2L2 



