MICROSCOPICAL AND PHYSIOLOGICAL EXAMINATION. 11 



thus established, the matter under consideration reached a 

 high state of development and great importance both for 

 science and for industry, due especially to the work of PASTEUR 

 and, subsequently, to other eminent scientists, who devoted 

 their energies to these investigations. 



(a) Sterilisation of glass and metal articles. 



Sterilisation properly so-called must always be preceded by 

 a thorough mechanical and, in many cases, also by a chemical 

 cleansing. Articles of daily use in the laboratory, as, for 

 instance, spatulas, pins, wire, etc., are heated directly in a flame 

 and allowed to cool in a germ-free space. Many pieces of 

 apparatus, however, do not admit of this treatment, and must 

 be sterilised either by heating in steam or in a water-bath, or 

 else in dry air by means of a sterilising oven, in which the 

 objects are heated for one or two hours at a temperature of 

 about 150 C. According to the nature of the objects, some 

 may be put directly into the sterilising oven, while others 

 must be previously wrapped in paper. The necks of flasks are 

 closed by cotton-wool, which should be covered by several 

 layers of filter-paper. 



(&) Sterilisation of nutritive liquids and solid nutritive 

 substrata. 



Nutritive liquids can be sterilised by filtration or by heat. 

 The former method presents the advantage that the liquids 

 treated undergo less change than when heat is employed, and 

 are, consequently, better suited for the development of many 

 species of micro-organisms, The necessary condition for 

 momentary sterilisation is, that the pores of the filter must be 

 smaller than even the smallest micro-organisms. Gypsum, 

 asbestos, charcoal, porcelain, and other substances have been 

 employed for this purpose, the liquids being forced by pressure 

 or suction through thick layers of these substances. The forms 

 most generally used are the Chamberland porcelain filter and 

 the silicious mask filter, made of calcined infusorial earth ; 

 these filters, however, require constant cleansing, and must also 

 be frequently sterilised, it having been proved that the bacteria 



