. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . riment. Inorder that the lime, or baryta water, shall be kept quitepure, and constant in strength, it will be necessary toremove the carbonic acid from the air which enters thestore bottle when some of its contents are withdrawn —as by making it pass through the pumice stone mois-tened with caustic potash. The accompanying figure will CARBONIC ACID. 201 serve to demonstrate how this can be readily effected :—A large glass store
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . riment. Inorder that the lime, or baryta water, shall be kept quitepure, and constant in strength, it will be necessary toremove the carbonic acid from the air which enters thestore bottle when some of its contents are withdrawn —as by making it pass through the pumice stone mois-tened with caustic potash. The accompanying figure will CARBONIC ACID. 201 serve to demonstrate how this can be readily effected :—A large glass store bottle is represented fitted with aglass tap to draw off the clear lime-water; any air whichenters must pass through the U-shaped tube, which ispacked with the pumice moistened with caustic potash. The turbidity which appears in the clear lime-wateron the addition of the oxalic acid solution, is due to theinsoluble oxalate of calcium being formed. If clear and pure baryta water is substituted for thelime-water, it will be found to absorb the carbonic acidmuch more readily than the latter, and the air sampleneed only be exposed to its action for an hour. The. Fig. 32.—iStore bottle for lime-water. strength should be about 7 grammes of the crystallisedhydrate to the litre of distilled water. A simple indicator of the exact stage at which neu-trality is reached, is a drop or two of a solution ofmethyl-orange, which then loses its yellow colour, andassumes a reddish hue. Conclusions to he drawn from the amount estimated.—It isgenerally agreed that the air of compartments, whichare occupied by human beings, should not show morethan o-o6 per cent, of carbonic acid, , the 0*04 per exists normally in the air, and 0*02 per cent, of allowable respiratory impurity. This limit is chosen 202 LABORATORY WORK. because, when it is reached solely from respiration, theorganic matter, &c., is judged to have reached an un-healthy amount, and is said to be appreciable to thes
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