Carbon Dioxide Apparatus Iii.’ Another Special Apparatus for the Estimation of Very Minute Quantities of Carbon Dioxide. By

For the purpose of estimating a very small amount of carbon dioxide in general biological problems, two pieces of apparatus have been already described.* The principle of the apparatus is as follows : 1. Exceedingly minute quantities of carbon dioxide can be precipitated as barium carbonate on the surface of a small drop of barium hydroxide solution. 2. When a drop of barium hydroxide is exposed to any sample of gas free from carbon dioxide it remains perfectly clear, but when more than a quite definite minimum amount of carbon dioxide is introduced a precipitate of carbonate appears, detectable with a lens. 3. Ry determining, therefore, the minimum volume of an? given sample of a gas necessary to give the first visible formation of the precipitate, its carbon dioxide content can be estimated accurately, since this volume must contain just the known detectable amount of carbon dioxide. In order to determine this minimum volume of the gas in the respiratory chamber, it has been recommended that in the case of biological problems,when the specimen gives off carbon dioxide continuously, and sometimes at different rates, varying wit’h the time, it would be much simpler not to attempt to determine the minimum