EXPANSION OF SOLIDS WITH RISE OF TEMPERATURE. 23 



of the temperature by the older observers hardly warrants us in going 

 further. But even with accurately observed temperatures the numbers 

 obtained to four places would not be the same for different specimens of 

 a substance, so much does the expansion depend on physical condition. 

 This is well illustrated by the difference between tempered and un- 

 tempered steel in this table, and is also shown by the difference 

 between soft iron and iron passed through a draw-plate. Comparing 

 the results of Lavoisier and Laplace with those of Ramsden, prieviously 

 given, for substances with the same name, the difference is still more 

 striking. In fact, we can hardly trust the generality of the results in 

 the tables beyond two figures ; and if greater accuracy is required, the 

 expansion must be determined for the particular specimen concerned. 



Fizeau's Method. A very remarkable and accurate method has 

 been devised by Eizeau, in which use is made of Newton's rings. It is 

 well known that, if a slightly convex lens is placed on a flat glass plate, 

 several bright rings are seen round the point of contact, these being due 

 to the interference of the light reflected upwards at the curved glass 

 surface with that which, passing down through the film of air, is 

 reflected back at the lower plane glass surface. If, instead of white 

 light, we use the nearly homogeneous light from a sodium flame, the 

 number of rings is very greatly increased, and they extend, in general, 

 to the edge of the lens. 



lei 



o 



i 



, ; ,~jl 



w 



i ( 'iG. 15. Diagram of Fizeau's Expansion Apparatus. B, block of which expansion 

 is to be measured placed on table T ; ss, levelling screws, also supporting lens, 

 L just above B ; /, film of air-forming interference bands ; P, right-angled 

 prism throwing light down and reflecting it out again ; Na, sodium flame behind 

 the plane of the figure ; m, mirror in the plane of the figure reflecting the sodium 

 light towards P ; tel, telescope directed towards P and viewing the interference 

 bands ; F represents the appearance of the bands in the telescope, which is 

 provided with cross- wires. The expansion table is protected by a case which 

 can be heated. This case is not represented. 



Now if the path of the rays of light is normal to the lower glass 

 plate, each concentric bright ring from the centre outwards cor- 

 responds to a thickness of the film of air - greater than the preceding 



