. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . ins itself. Hesses apparatus is seen by figure 37 to consist of along glass tube connected at one end to the aspirator;the small India rubber cap which closes the other endis removed just before use, and 50 of pure glycerineis poured into the tube, which is then turned round onits horizontal axis so as to make the glycerine coat thewhole interior. As the air is subsequently aspiratedthrough the tube, the suspended matter is
. Public health laboratory work, including methods employed in bacteriological research, with special reference to the examination of air, water and food contributed . ins itself. Hesses apparatus is seen by figure 37 to consist of along glass tube connected at one end to the aspirator;the small India rubber cap which closes the other endis removed just before use, and 50 of pure glycerineis poured into the tube, which is then turned round onits horizontal axis so as to make the glycerine coat thewhole interior. As the air is subsequently aspiratedthrough the tube, the suspended matter is caught up bythe glycerine—which can be removed and examinedmicroscopically. SUSPENDED MATTER IN THE AIR. 227 Methods in which glycerine is employed are some-what unsatisfactory, since it is rare that the original pure glycerine will not contain marked evidence ofsolid particles. A preliminary microscopic examinationof the glycerine would not, however, entail much addi-tional labour or loss of time ! A third plan entails the use of a pure sugar filterthrough which the air is slowly drawn; the sugar is thendissolved in a sufficiency of pure water, and the sus-. FiG. 37.—Hesses apparatus for the collection of suspended mattersin the air. a. The extremity connected with the aspirator, h. A re-moveable cap. pended matters caught up in it are retained in sus-pension in the water, when they may be collected andexamined microscopically. The filter is best arranged as a glass tube, at least aninch in diameter, disposed horizontally, and packed—not too tightly—for several inches with the sugar crys-tals. One end of the tube is left open for the entranceof air, and the other connected by india-rubber tubingwith an aspirator. The filter dissolved—the suspended Q2 228 LABORATORY WORK. matter may be separated by filtration through a weighedSwedish filter-paper, then dried at a low temperatureand weighed. If the amount of air aspirated has beenrecorded, the weighed quantity of its
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