Hygiène scolaire . correspond à 1 mg de CO 2 et 1 mg de CO 2-réponse à 0,5 ccm. De CO 2, Donc 18,4X1 ing.=18,4 mg. Ou 18,4X0,5=9,2 ccm. D'acide carbonique in5,168 ccm. D'air, le contenu de la fiole. Pour findla quantité d'acide carbonique par mille ccm. Ofair, la proportion suivante est utilisée: 5,168 : 9,2=1,000 : X. A Partir de là, on obtient a;:=f|-f|-=:1,78 parties permille. 76 YEKTILATIOK Bien que Pettenkhots soit la seule méthode exacte, et donc la seule disponible pour les enquêtes scientifiques, néanmoins, le processus mini-métrique beaucoup plus simple et plus rapide de Smith-Lunge ^ est suffisant

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School hygiene . ercorresponds to 1 mg. of CO2 and 1 mg. of CO2 cor-respond to 0.5 ccm. of CO2- Hence 18.4X1 ing.=18.4 mg. or 18.4X0.5=9.2 ccm. of carbonic acid in5, 168 ccm. of air, the contents of the flask. To findthe amount of carbonic acid per thousand ccm. ofair, the following proportion is used: 5, 168 : 9.2=1, 000 : X. From this we get a;:=f|-f|-=:1.78 parts perthousand. 76 YEKTILATIOK Though Pettenkoffers is the only exact method, andtherefore the only one available for scientific investiga-tions, nevertheless, the far simpler and quicker mini-metric process of Smith-Lunge ^ is sufficient for purelypractical purposes. The principle of this is to intro-duce known quantities of the air to be tested into agiven quantity of baryte solution till it becomes-cloudy. The more quickly this happens, the less car-bonic acid there is in the air—the more slowly, the more. The Smith-Lunge method requires a round bottle*3.8 ccm. in diameter, and 9 cm. high, having a capac-ity of about 53 ccm. (see figure 5).. Fig. 5. THE SMITH-LUNGE AIR TESTER Two tubes pass through holes in the stopper. One?(be) is straight and extends to the bottom; the other (de) ^Georg Lunge, Zur Frage der Ventilation undBeschreibung des minimetrischen Apparates zur Bes-timmung der Kohlensaure. Zurich, 1877. SMITH-LUi^GE METHOD 77 which is bent at right angles and barely passes throughthe stopper, has the rubber tube (d f g h i), 20 to30 cm. long, with the pearshaped bulb k of about 28ccm. capacity, attached. In this rubber tube there isat (f) a longitudinal slit 1 cm. long, —which acts as ayalve. If the tube (a b) is pressed together with one hand, and the rubber bulb k with the other, the air in thelatter will escape through the slit (f). The bulb nowexpands through its own elasticity and fills with airdrawn through the bottle by means of the tube (ac), which is now left open. After the test bottle has thusbeen filled several times with the air to be examined, 7 ccm. of a solution of 6 g. of barium hyd

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