I02 EXAMINATION OP AIR AND WATER 



2. Miquel's method (modified)— 



The method of Miquel is the one which, for quantitative 

 analysis, has given the best results in our hands, and this method 

 will be found most reliable except when the air is very heavily 

 charged with moisture. Advantage is taken of the filtering pro- 

 perties of an inert salt in a finely granulated condition, possessing 

 no antiseptic power, and unalterable and nonfusible at a tempera- 

 ture of 1 70°- 1 80° C. Such salts were found by Miquel, after a long 

 series of experiments, to include the sulphates and phosphates 

 of sodium, and the sulphate of magnesia, all of which were found 

 to favour rather than to retard the growth of organisms. Of these 

 three salts, sulphate of sodium was found to be the best, as being 

 more soluble than the phosphate, and less deliquescent than the 

 sulphate of magnesia. To prepare the filtering medium, a suffi- 

 cient quantity of the salt should be thoroughly dried, and then 

 reduced to coarse powder in a mortar. The powder thus obtained 

 is then thrown upon a sieve containing between 15 and 18 meshes 

 per cm. That which passes this sieve is then thrown upon a 

 second containing between 22 and 25 meshes per cm. The finer 

 particles of the powder pass through this latter mesh, leaving upon 

 the sieve an evenly granulated powder of which the individual 

 grains will be found to be of about -5 of a mm. in diameter. 

 A sufficient quantity of the powder should be prepared, and if 

 stored in a dry place, and in a carefully stoppered bottle, will be 

 found to remain unchanged for a lengthened period.^ 



The filtering apparatus itself is of the simplest description, and 

 is represented by Fig. 30. It consists of a piece of stout glass tubing 

 about 12 cm. in length and -75 cm. internal diameter, constricted 

 in the middle as shown. At one side of the constriction a light plug 

 of glass wool ia) is inserted in order to act as a support to the granu- 

 lated salt used for the filtration. The filtering substance is shown 

 at b, and consists of the granulated sulphate of soda, prepared as 

 explained above. The extremities of the tube are plugged with 

 asbestos plugs {c and d) to prevent contamination of the tube after 

 sterilisation. It will be found convenient to prepare a number of 

 these tubes at a time. The central plug of glass wool should be 

 inserted and the tube itself heated in a Bunsen flame until all trace 

 of moisture has disappeared. About i gramme of the pulverised 

 salt in a thoroughly dry condition is then introduced into the tube 



^ Messrs Baird & Tatlock of 14 Cross Street, Hatton Garden, E.G., will 

 supply on order this salt in a properly granulated condition. 



