A chemist has a 10-gallon sample of river water taken just downstream from the outflow of a chemical plant. He is concerned about the concentration, c (in parts per million), of a certain toxic substance in the water. He wants to take several measurements, find the mean concentration of the toxic substance for this sample, and have a 95% chance of being within .5 part per million of the true mean value of c. If the concentration of the toxic substance in all measurements is normally distributed with σ = .8 part per million, how many measurements are necessary to achieve this goal?