Correction may be required for variations from the normal pressure of 760 mm. and normal temperature of 0° C., in accordance with ordinary rules. =In Lunge and Zeckendorf’s Method=, the air to be examined is pumped through a glass bottle in which is 10 c.c. of a N/500 solution of Na₂CO₃ containing phenolphthalein as an indicator. The air is pumped by a hand pump through this solution until the phenolphthalein is decolourized. The number of times the ball of the pump has been squeezed indicates the amount of CO₂ present in accordance with a table prepared from separate experiments by Pettenkofer’s method. =Dr. Angus Smith’s plan= for the estimation of carbonic acid in air is similar in principle to the last calculations. It is based on the fact that the amount of carbonic acid in a given volume of air will not render turbid a given amount of lime water, unless the carbonic acid is in excess. TABLE.—_To be used when the point of observation is “No precipitate.”_ Half an ounce of lime water containing ·0195 gramme lime. AIR AT 0° C. AND 760 M. M. BAROMETRIC PRESSURE. ┌───────────────┬────────────────┬─────────────────┬─────────────────┐ │ CARBONIC ACID │VOLUME OF AIR IN│SIZE OF BOTTLE IN│SIZE OF BOTTLE IN│ │ IN THE AIR │ CUBIC │ CUBIC │ OUNCES │ │ PER CENT. │ CENTIMETRES. │ CENTIMETRES. │ AVOIRDUPOIS. │ ├───────────────┼────────────────┼─────────────────┼─────────────────┤ │ ·03 │ 571 │ 584 │ 20·63 │ │ ·04 │ 428 │ 443 │ 15·60 │ │ ·05 │ 342 │ 356 │ 12·58 │ │ ·06 │ 285 │ 299 │ 10·57 │ │ ·07 │ 245 │ 259 │ 9·13 │ │ ·08 │ 214 │ 228 │ 8·05 │ │ ·09 │ 190 │ 204 │ 7·21 │ │ ·10 │ 171 │ 185 │ 6·54 │ │ ·11 │ 156 │ 170 │ 6·00 │ │ ·12 │ 153 │ 157 │ 5·53 │ │ ·13 │ 132 │ 146 │ 5·15 │ │ ·14 │ 123 │ 137 │ 4·82 │ │ ·15 │ 114 │ 128 │ 4·53 │ │ ·20 │ 86 │ 100 │ 3·52 │ │ ·25 │ 69 │ 83 │ 2·92 │ │ ·30 │ 57 │ 71 │ 2·51 │ └───────────────┴────────────────┴─────────────────┴─────────────────┘