Lecture VIII. 59 



in which all the vegetative cells and spores had been killed by the 

 simple ruse described by Tyndal as discontinuous heating. 



In this method the vegetative organisms are killed by being 

 exposed to a temperature of 75 to 100 C. Then according to 

 observation there are left uninjured only certain spores which will 

 be capable of germination and growth. These are induced to 

 germinate by the restoration of favourable conditions and when 

 they have entered upon their vegetative condition, the tempera- 

 ture is raised once again and catches them in their susceptible 

 condition. Lest the timing of the second heating might have 

 been unfortunate and have found some cells in the resistant 

 state, a second restoration of moderate temperature for some 

 hours and a third heating is arranged. Organic substances thus 

 treated with discontinuous heat and kept free from external in- 

 fection do not decay, do not produce bacteria nor do they give 

 any indication that spontaneous generation has occurred within 

 them. They are said to be sterilised. And countless such 

 sterilisations made daily attest the absence of proof for such 

 spontaneous generation as was formerly believed to be of common 

 occurrence. The sealing to prevent external infection during and 

 after sterilisation was first effected by using glass vessels and 

 closing the openings by melting the glass. This method has 

 been almost entirely replaced by the use of closely packed plugs 

 of cotton-wool, which are found almost equally effective. They 

 act by filtering out the dust and consequently the dust-borne 

 bacterial cells and spores. The usual apparatus in which the 

 process of discontinuous heat is applied to culture media is the 

 " steamer ". It has the form of a cylindrical metal vessel pro- 

 vided with an internal shelf on which the vials containing the 

 culture medium may stand. Below the shelf is water, and the 

 vessel is closed above by a lid with a small aperture to allow the 

 escape of steam generated from the water when the vessel is 

 supported over a burner. When thus arranged the whole 

 vessel becomes filled with steam and the vials containing the 

 medium are quickly raised to the temperature of steam. 



Sometimes the " steamer " is nowadays replaced by an 

 " autoclave " which is essentially similar in its design, save that 

 it closes hermetically and is made so strong in the walls that it 

 is possible to generate considerable steam pressure in it without 

 danger. The pressure is regulated by a safety valve. As the 

 pressure of the steam rises, the temperature rises also ; in this 

 way the culture medium within is exposed to a temperature 

 which will kill, not only the vegetative cells but also the spores. 



