OF ARTS AND SCIENCES. 66 



ami /„ — < = 100 — 31.3 = G8.7 ; 

 .-. A'=.133G. 



[^xy]x^2 = 0-097 ; .-. Drt,-\r Drt, = .217, 



and <o — < = 1 00 — 3 1 .8 = 68.2 ; 



.-. A'=:.r292. 



[^xy]x^3 = 0.100; .'.Drt,-\-Drf, = .22, 



and /„ — ; = 100 — 32.3 = 67.7 ; 



.•..fi:=:.132. 



[AyL=4=-^03; .•.i)r/^ + Z)r/, = .233, 



aud ^0 — < = 100 — 32.8 = 67.2 ; 



.•.Ar=.1348. 



[i),y], = 5 = .094 ; .' . D^t.^ Drt, = .214, 



and <g — i = 100 — 33.3 = 66.7 ; 



.•.ir=.1303. 



The average value of K is found, from the above observations, to be 

 K— .1353. 



The conductivities deternained above should all be the same, and 

 they are almost exactly the same, except the first ; for after the first 

 hour the sand must have reached a permanent state of heat, and the 

 mean temperature was sensibly the same for all the values of K; the 

 water only gained 2°. 6 of temperature. Consequently, from these re- 

 sults it cannot be determined whether the conductivity of sand in- 

 creases or decreases as the temperature increases. But to find how 

 the conductivity varies with the temperature, the value of t^ should be 

 changed ; therefore the temperature in the test-tube should be varied. 



To reduce the average value of K to the same units used in glass, 

 we multiply by .000035 and .0328 and divide by .00107, or we merely 

 multiply by .00107 and find as the value of K, .000144771. 



Forbes determined the conductivity of sand by burying thermome- 

 ters below the surface (3 — 25 ft.), and found it to be .000169. The 

 temperature of the sand for Forbes's conductivity was about 62° F. 

 = 34°. 4 C. The mean temperature of the sand in my experiment 

 was about 50° C. From this it is probable that the conductivity of 

 sand, like that of glass and of iron, decreases as the temperature in- 

 creases. The specific gravity of the sand in my experiment was 16. 



The errors arising from values of D^y are probably not very large. 

 The arc of the first curve is almost a straight line, and the values of 



