Let us try a 10 per cent. solution of salt. Suppose the maximum current this will carry is 1/4 ampere per square inch, which will give a cross section of the solution of at least 60 / 1/4 = 240 square inches. Now, the specific resistance per inch cube (i.e., the resistance between two opposite surfaces of a cube whose side measures 1 inch) of the 10 per cent. solution of salt used in test No. 3 was 2.12 ohms. The drop, CR, will be 2.12 x 1/4 = 0.53 volt per inch length of solution between electrodes. Hence, the electrodes will have to be 40/0.53 = 75 inches apart. This would require about three barrels connected in series. This was taken merely as an illustration, because its specific resistance was known when the current density was 1/4 ampere per square inch. This solution, however, will carry safely 1/3 ampere per square inch, but I used the previous figure, since I did not know its specific resistance for this current density, because its specific resistance will be lower for a larger current density on account of the higher temperature which it will have, for the resistance of a solution decreases as its temperature increases. To reduce this length would require a solution of higher specific resistance, that is, a solution containing less than 10 per cent. of salt, and an increase in the cross section, since the maximum carrying capacity also diminishes as the percentage of salt diminishes. Only approximate calculations are useful because variations in temperature, amount of salt actually in solution and the rate at which heat can be radiated, all combine to give results which may vary widely from those calculated. As a matter of fact, it is seldom necessary or advisable to use a solution containing over 2 or 3 per cent. of salt. The best way to add salt to a liquid rheostat is to make a strong solution in a separate vessel and add as much of this solution as is needed. This avoids the annoying increase in conductivity of the solution which happens when the salt itself is added and is gradually dissolved. Liquid rheostats are ever so much more satisfactory for alternating than for direct current testing. The electrodes and solution are practically free from decomposition, and a given cross section seems to be able to carry a larger alternating than direct current--probably due partly to the absence of the scum on the surface which hinders the radiation of heat. * * * * * THE PROGRESS OF MEDICAL EDUCATION IN THE UNITED STATES.