456 Prof. R. Threlfall on the Measurement 



different state of strain to the interior, might be approximately 

 removed. The plates were planed first on one side and then 

 on the other till the right quantity of metal was removed ; 

 the last cuts being taken very fine. The handle affixed to 

 the upper plate was of course arranged so as to fit loosely, 

 and not in any way constrain the free expansion and con- 

 traction of the plates ; the temperature having been often 

 violently changed (by heating for cement &c), it is hoped that 

 the plates may be considered fairly well annealed. There is 

 no doubt, however, that for complete satisfaction in an abso- 

 lute measurement the plates should be capable of being 

 optically examined during the process of heating. This 

 would require to be done in the gas-oven or other uniform 

 field of temperature and at the time the experiments were 

 made. I had not the requisite appliances. 



It was found by several trials that the best way of obtaining 

 a layer of gum free from bubbles between the plates, was to 

 float the lower plate or tray pretty full of gum, and also to 

 obtain a layer of gum free from bubbles and in quiet fusion 

 on the upper plate. In the case of the grass-tree gum this 

 could only be obtained by heating the gum for some time at 

 a temperature higher than the one at which it was when the 

 plates were brought together. This was accomplished much 

 in the same way as is sometimes done in microscopy — when 

 the cover-slip is placed on the slide with one edge down and 

 the other end gradually lowered. The freedom from bubbles 

 of the layer of gum obtained in this way was tested by allow- 

 ing the plates to cool, and then heating the lower one till its 

 surface reached the temperature of the melting-point of the 

 gum, the upper one being kept cool. This being done the 

 upper plate could finally be lifted, leaving only a small portion 

 of the gum on the lower plate — owing to the small heat-con- 

 ductivity of the gum. The layer of gum was left thick on 

 one occasion for the purpose of this test, and when the 

 manipulation described above was properly carried out there 

 were no bubbles ; the layer in fact was very homogeneous 

 indeed. The only danger left so far as the insulating material 

 is concerned is that it may tend to crack away from the 

 plates during cooling. In the cases examined this did not 

 seem to be the case, because in the first place a thick layer of 

 the same substance rapidly cooled on a thin plate showed no 

 tendency to crack. Again, a great many insulating substances 

 are more or less plastic, or rather viscous, down to tempera- 

 tures very nearly approaching those at which experiments 

 are usually made (in this country 23° C. is a not unusual 

 temperature). Again, the massiveness of the plates being 



