_Determination of Aqueous Vapour._ To effect this an aspirator must be used (see ASPIRATOR). This instrument is easily made, and is not expensive. The accompanying figure will illustrate the arrangement generally adopted: _a_ is an aspirator made of galvanised iron or sheet zinc. It holds from 50 to 200 litres (from 11 to 44 gallons). By this means a known volume of air is drawn through the tubes marked _b_, _c_, _d_, _e_, which may be filled with pumice-stone moistened with strong sulphuric acid; but if the carbonic acid is to be estimated as well, _b_ and _c_ are filled with moist hydrate of lime (potash used to be employed, but hydrate of lime is to be preferred, as the potash absorbs oxygen), and _d_ and _e_ as above. Each of the tubes is accurately weighed previously to connecting them with the apparatus. [Illustration] It is imperative to have each of the tubes connected by perfectly air-tight joints. The gain of weight in _d_ and _e_ gives the water in _b_ and _c_ the carbonic acid. _Determination of Carbonic Acid._ A better and perhaps more exact means of determining the carbonic acid is that invented by PETTENKOFER. It may be briefly described as follows:——Baryta water of definite strength is prepared and accurately standardised by a standard solution of oxalic acid. A portion of this baryta water is then made to act upon a definite quantity of air. It will absorb the whole of the carbonic acid in that air. The alkalinity of the liquid will in consequence be diminished; it will take less of the oxalic-acid solution than before, which shows so much less caustic baryta, and from which the carbonic acid absorbed may be easily calculated. _The actual Analysis._ Two kinds of baryta water may be used, the one containing 7 grammes to the litre, the other three times that strength; 1 c. c. of the stronger = 3 m. grms. of carbonic acid; 1 c. c. of the weaker = 1 m. grm. The baryta water is best kept in the bottle represented below. [Illustration] The bottle (_a_) contains the baryta water. It has an accurately-fitting double-perforated stoppered caoutchouc. The left-hand tube is connected with the tube (_b_) containing pumice-stone moistened with potash, while the right-hand one is a syphon. When required for use the stop-cock (_f_) is opened, and suction applied by a glass tube to F. The syphon is thus filled and the stop-cock closed. If a pipette is required to be filled its nozzle is inserted at F, the stop-cock compressed, and the fluid immediately rises into the pipette. The air entering the bottle as the fluid decreases in _a_ is, of course, thoroughly deprived of its carbonic acid by the tubes at _b_. The first thing to be done is to standardise the baryta solution by a solution of oxalic acid, containing 2·8636 grammes of crystallised oxalic acid to the litre.