It is found in practice, however, that the conductivity of salts agrees with the numbers deduced from the velocity of their ions only when the solution is a very dilute one, and even then not always. This can be ascribed to either or to both of two causes. If the solution is a strong one, the molecules of salt may bear an appreciable ratio to the molecules of water, and may interfere by their possibly greater friction with the free transit of the ions. Or, on the other hand, some of the molecules may not be resolved into ions, and there may be fewer " boats " to carry across the electrons, the progress of which towards the anode and the kathode must consequently be slower, for the non-ionised molecules take no share in the conveyance of electricity. And as the conveyance of electricity depends on the number of ions and on the rate at which they move, if the latter is known, the relative number of ions may be calculated from measurements of the conductivity of the solution.