A still more striking experiment may be made by exposing the intestines and pouring a small quantity of sodium citrate solution on their peritoneal surfaces. Immediately they become extremely active. If now they be washed off with a little NaCl solution and a few c.c. of a solution of calcium chloride be poured on them they will come to rest at once. These solutions must be at body temperature and isotonic with the blood. If the loops which have been quieted by CaCl_{2} are again moistened with the citrate solution they will be set into motion as before, and a subsequent application of CaCl_{2} will again cause all movement to cease. This may be continued almost indefinitely. I have set the same loops in motion and stopped them by these solutions as many as sixteen times in succession. Magnesium chloride acts in this respect like calcium chloride, and a similar but slighter action is possessed by strontium chloride. Magnesium sulphate has a purgative action just as many other sulphates have. Magnesium citrate also acts in the same way as other citrates. The action of the magnesium in these cases seems to be subordinate. In addition to the inhibitory action of calcium and magnesium on the peristaltic movements of the intestine, these substances also suppress the secretion of fluid into the intestine. This is shown in the previous chapter (IV), where tables are given to show the course of the experiments. According to these experiments, the normal rate of secretion in an isolated loop was measured. The quantity of fluid secreted was small, but the application of CaCl_{2} or MgCl_{2} to the serous surface of the loop stopped the secretion entirely. The subsequent application of sodium citrate caused it to flow again. In its counteraction of the effect of saline purgatives, calcium behaves in the same way. The increased peristalsis or secretion caused by sodium citrate, sulphate, etc., is entirely suppressed by the administration of calcium or magnesium chloride. This is not true to the same extent of those activities produced by barium chloride. CaCl_{2} as far as I have been able to determine only partially counteracts the effect of BaCl_{2}. That an antagonism does exist cannot be doubted, but the violent peristaltic movements brought about by BaCl_{2} cannot be fully suppressed by CaCl_{2} or MgCl_{2}. As mentioned above, BaCl_{2} in extremely small doses causes an increased flow of urine.[55] This can be partially inhibited by CaCl_{2}. With slightly larger doses of barium, the flow of urine often ceases abruptly, due probably to the contraction of the musculature of the urinary passages, or possibly to a contraction of the blood vessels of the kidney. This condition is relieved by the administration of CaCl_{2}, that is, the calcium merely counteracts the effect of the barium on the musculature of the urinary passages or blood vessels, whichever it may be.