284 Mr. G. F. Rodwell on the Effect of Heat on 



Eods of other metals gave the following results : — 



Expansion-apparatus. Various observers. 



T r^nnAiiAOPc r -00001182 

 -000011025 {.00001194 



r -00001666 



Copper -0000174433 \ -00001678 



[ -00001722 



Lead -0000302121 -00002924 



Zinc -0000288761 -00002918 



It is thus abundantly evident that the apparatus is capable of very con- 

 siderable accuracy, especially when we remember that we are dealing 

 with rods of metal only six inches long. I venture to think that such an 

 apparatus would prove a useful adjunct to a Physical Laboratory ; for it 

 would not only afford a means of determining coefficients of expansion 

 with rapidity and accuracy, but it might be used for determining thick- 

 nesses in thousandths and ten-thousandths of an inch. Eor if the 

 substance were introduced between the agate face of the micrometer- 

 screw and the glass rod H, and if the index were then driven round to a 

 certain point (say 20° of arc), and so with the other substances tested, an 

 exact uniformity of pressure of the face of the screw on the substance 

 would be secured, and minute thicknesses might thus be measured with 

 accuracy. 



In constructing such an apparatus, extreme rigidity of the fixed parts 

 is a main necessity ; the base should be of thick marble, and the micro- 

 meter-screw support should be deeply let into it and firmly secured. 

 Massive supports, apparently fixed with the utmost firmness, sometimes 

 yield with surprising readiness to the extent of one five-hundredth of an 

 inch. The elasticity of the metal sometimes causes great inconveniences. 

 The levers must be firm and strong, but not unnecessarily heavy ; they 

 should work very smoothly. Agate should be let into the brass at the 

 points of contact. Care should be taken to avoid any transference of 

 heat from the hot trough to other parts of the apparatus. By the use of 

 a paraffine of high boiling-point, the temperature of a bar submitted to 

 heat in the trough (F) can be raised to 300° C. Under such circum- 

 stances great care is requisite to avoid heat from being radiated or other- 

 wise communicated to other parts of the apparatus. 



2. Means employed to determine tlie expansion in passing from tlie solid 

 to the liquid condition. 



The determination of the amount of expansion which iodide, chloride, 

 and bromide undergo in passing from the solid to the liquid condition 

 was effected in the following manner: — A conical tube of platinum 

 weighing 44*844 grammes was carefully filled with mercury at a 

 known temperature. It contained 105-43 grammes of mercury. It 

 was then filled with the substance under examination at its melting- 



