1. Suspensions of manganese dioxide given intravenously to the cat are virtually eliminated from the body in 1 week (168 hours).

2. As this process of removal goes on, there is a transfer of manganese from lungs to liver.

3. Similar suspensions of manganese dioxide given intravenously to rabbits are excreted more rapidly, owing to the fact that in these animals the lungs are not involved and the whole process of removal from the blood is carried on by the liver.

4. A method for the quantitation of rhodonite (manganese metasilicate) is given.

5. Three different suspensions of rhodonite, varying in the size of the largest particles from 1.3 to 0.2µ, when injected intravenously into cats show a greater and greater tendency toward arrest in the liver as the size of the individual particles becomes smaller.

6. When rhodonite suspensions containing particles averaging 1.1µ in diameter are injected intravenously into cats, the primary sites of deposition are similar to those observed for manganese dioxide suspensions similarly injected. The disappearance of rhodonite is exceedingly slow and is apparently completed in about 50 days following injection.

7. Rhodonite should prove a suitable substance with which to attempt the production of lung fibrosis in cats through the medium of intravenous injection. Manganese dioxide, while a useful agent for studying the immediate sites of localization of particulate material carried by the blood, lacks permanency after deposition and is consequently unsuitable for studies upon chronic changes induced by foreign substances.

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