COAL-TAR AND WATER-GAS TAR CREOSOTES. 53 



gravities of oils, and at best the coefficient of expansion can be only 

 a rough average of the various coefficients of expansion at different 

 temperatures for different oils. In practice, it is usual to say that 

 the change in volume is 1 per cent of the original volume for every 

 22.5° F. of temperature change. This factor is approximately 

 accurate and is based on the fact that creosote oil changes its specific 

 gravity roughtly 0.0008 per degree centigrade, or 0.00044 per degree 

 Fahrenheit. For commercial work over a short range of tempera- 

 ture, this factor is probably sufficiently accurate; but for refined 

 work, such as the experimental determination of absorption of creosote 

 by wood, the figure of the next decimal place should be known. 

 Some work at the Forest Products Laboratory shows that the figure 

 derived from the change of gravity per degree centigrade lies between 

 0.00077 and 0.00078, and between 0.000428 and 0.000433, with an 

 average of 0.000430 per degree Fahrenheit. This work also shows 

 that the actual change in gravity is independent of the initial gravity, 

 provided the initial gravity at 10° C. is between 1.01 and 1.05; 

 therefore a change in volume is dependent upon the initial gravity of 

 the oil; that is, an oil with a specific gravity of 1.01, when heated 

 through 100° F., will not have the same increase in volume as will 

 an oil of 1.05 specific gravity heated through the same temperature 

 difference. The volume, however, may be calculated by the use 

 of the formula 



GV 



V' = 



G-(T-T') 0.00077 



where T and T' are temperatures in degrees centigrade; or by the 

 use of the formula 



r ^ 



G—{T—T') 0.00043 



where T and T' are the temperatures in degrees Fahrenheit. In 

 these formulae, V is the volume at the temperature T' , F is the 

 volume at temperature T, and G is the specific gravity at temper- 

 ature T. 



VISCOSITY. 



Viscosity is a measure of the inner friction of liquids, that is, the 

 friction produced by the liquid moving on itseK. There are no 

 instruments in commercial practice that measure this property 

 directly. Most commercial viscosimeters are so constructed that the 

 rate of flow of the liquid through an orifice of definite diameter under 

 a definite head may be accurately measured. This does not give 

 true viscosity, but does give an empirical measure of that property. 

 Nearly all instruments are standardized by water at a fixed tempera- 

 ture. The efflux time in seconds of the liquid under examination 

 divided by the efflux time of water at the standard temperature, is 



