Logarithms can probably be taught using logarithmic Cuisinaire blocks, which might be presented along with suitably scaled graph paper. (You remember Cuisinaire blocks from primary school - they're in lengths of 1, 2, 3... and they're all different colours.) I haven't tried this yet in teaching but it's the next thing I'll try. P.S. The primes assume their proper importance in arithmetic using logarithmic blocks.