Conductivities of Cements and other Substances. 501 



The thickness was obtained by measuring with a micro- 

 meter-gauge the distance between the two pegs, first, when 

 the plates were in contact with each other, and, secondly, 

 when the glass was between them. 



The following table gives the observations taken during 

 each experiment, and shows the method of determining the 

 conductivities from the observations : — 



Remarks on Specimens Experimented on. 



(a) The plates of Portland cement and plaster of Paris, &c, were made 

 by pouring the liquid cement on to a glass plate, on which three 

 equal beads strung together were laid, a second glass plate was 

 pressed down upon these beads, after a few hours the glass plates 

 could be removed and a plate of cement of uniform thickness was 

 obtained. 



The plate of plaster of Paris and sand consisted of two parts 

 by weight of plaster of Paris to one part by weight of sand. 



(b) When the substance experimented upon was a powder, the brass 



plates were kept apart by three stops of wood, and the powder was 

 prevented from falling out at the edges by a very narrow circular 

 ring of fibre. 



(c) The three values given for the conductivity of garden-soil refer to 

 three specimens of soil of the same kind but containing different 

 amounts of moisture ; the first value refers to dry soil, the second to 

 slightly moist, and the third to damp soil. The results show how 

 greatly a small amount of moisture affects the conductivity. 



(d) The calico in the first experiment was dry, and weighed 891 grams. 

 In the second experiment it was exposed for some time to a damp 

 atmosphere, and its weight increased to 906 grams, so that the con- 

 ductivity of calico increases 20 per cent, for 1-6 per cent, increase in 

 weight due to moisture absorbed. 



(e) In the three experiments on old flannel, the flannel contained different 



amounts of moisture. In the first experiment the flannel was quite 

 dry and weighed 8*21 grams, in the second it was exposed to a 

 damp atmosphere, and its weight was 8*29 grams, and in the third 

 experiment was damped with water and its weight increased to 

 9'05 grams. Thus the conductivity of flannel increased about 11 per 

 cent, for 1 per cent, increase in weight due to absorbed moisture, 

 and afterwards increased 25 per cent, more for a further increase 

 of 10 per cent, in weight. 



The flannel in the third experiment is almost as good a conductor 

 as dry calico. 



The new flannel, flanelette, silk, and linen were dry. 



