522 Mr. D. Tyrer on Relations between the Physical 



The thermal conductivity was measured by the slab method 

 — a slab of about 300 sq. cm. area and of a thickness of 

 •245 cm. being pressed between a steam and a cold-water 

 vessel. The temperatures of the cold water were read from 

 minute to minute as they rose. Careful corrections were 

 made for " radiation " loss from the cold vessel. Two distinct 

 sets of readings were taken. The values of the conductivity 

 obtained were "000150 and "000141 respectively. Taking 

 the mean of these and employing the value obtained experi- 

 mentally for the emissivity, viz. "000275, the critical radius 

 becomes "527 cm. The value read off from fig. 3 is about 

 *57. The agreement is as good as was to be expected. 



The conclusions to be drawn from these experiments are 

 that : 



(1) On narrow steam-pipes, up to about half a cm. ex- 

 ternal radius, the application of an asbestos coating encourages 

 the escape of heat. This radius is below what may be termed 

 engineering dimensions. But in experimental apparatus it 

 is not at all an unusually small radius ; and for such pipes 

 the application of lagging is a delicate question. 



(2) When the radius is much smaller than this critical 

 value, the coat must have an external radius much greater 

 than half a centimetre before the lagging efficiency is as 

 good as without a coat at all. Thus, the curve would seem 

 to show that the escape of heat is the same for external radii 

 of "24 cm. and 2*0 cms. 



ri LII. Relations beticeen the Physical Properties of Liquids at 

 the Boiling-Point. By D. Tyrer *. 

 SIMPLE relation exists between the latent heat of 

 vaporization of a liquid and the molecular volume at 

 the boiling-point. The relation may be expressed 



lm=kVv, 



where L is the latent heat of a liquid at its boiling-point, 

 V the molecular volume at the boiling-point, M the molecular 

 weight, and K a constant. This relation, which may for the 

 present be regarded as empirical, holds with a fair degree of 

 accuracy for most classes of substances. In the Table given 

 below the values of the molecular volumes are the mean 

 values of the results chiefly of Kopp, Pierre, Schifr", Thorpe, 

 Grartenmeister, and Young. In the last column are given 

 the latent heats calculated according to the above equation, 

 the value of K having been taken as 1583. This figure 

 is the mean value of K for the aliphatic esters, calculated 

 from the results of Schiff and J. C. Brown. As Brown's 

 * Communicated by Prof. H. B. Dixon, F.R.S. 



A 



