1. When copper chloride is allowed to act on Aspergillus niger there is at first a period during which there is no change in the rate of the production of carbon dioxide, following which the rate of respiration falls. The interval of no change is called the latent period.

2. When the copper is removed from the external solution before the end of the latent period this interval is prolonged. The rate of respiration then falls to a new level below the normal level.

3. Experiments on Nitella and on Valonia indicate that the copper penetrates the cell almost immediately.

4. The length of the latent period varies inversely as a constant power of the concentration of the copper.

5. These results are explained by assuming that the copper is made active in the respiration system by means of a reversible reaction. By using appropriate velocity constants the experimental curves can be duplicated by calculated curves.

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