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Over the last 100 years (I am taking some licence here because some sort of capacitors did exist back in 1900) there have been many remarkable changes in capacitor construction. Types which were in vogue say 25 years ago are no longer satisfactory for the purpose required. One such type would be the silver-mica which was all the rage for frequency determining components in radio. Today I wouldn't consider it for a second because modern ceramic types are much, much more stable with well defined temperature coefficients and tolerances. Similarly the modern polystyrene types are vastly improved.
Here is a listing of some of the more common types and their properties.
Quality air variables are rarely used today principally because of the costs involved. New air variables are almost prohibitively expensive for amateur radio or electronic hobby use. There is a large market of surplus or second hand types but even still they are relatively expensive and usually require a further expensive reduction drive and dial mechanism.
These today have largely been superseded by voltage variable diodes, frequency synthesisers and digital read outs.
Trimmer air variables have been replaced by more economical and miniature ceramic or plastic film types with a variety of ranges.
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